After reading some contributions here, I recalled 4 more losses:
Fishing rod top endcap in my first activation of Mt. Etxauri EA2/NV-070. Solution applied here: SOTApole end caps - #25 by G4OIG
(&3) A fleece and a fishing rod ring during the ascent to Mt. Belabartsaitsa EA2/NV-044. The fleeze was replaced by a brand new one and the ring was never replaced. I don’t need it anymore.
Sunglasses during the ascent to Mt. Itzaga EA2/NV-029. Never replaced so far.
73,
Guru
Amazingly whilst I seem to have had a rather mixed record at arriving on the summit with the wrong kit - for instance the mains battery charger but not the battery 
I have not managed to leave much behind. However I can still sometimes be found by the puddle at the trig point on Hoove (G/NP-024) looking for the tiny screwdriver attachment for a Leatherman last seen 3 years ago!
There’s one of mine near the top of Grassmoor. The tape came off when I wasn’t looking 
I’ve also lost 2 pieces of pvc pipe somewhere between Pillar and Kirk Fell. these were for my MFD that I’ve had for some time.
Other people have lost a fair number of sandwiches to my dog. He’s like lightning when there’s a careless waving of a sandwich. I apologise a lot.
On a sad note I pick up a lot of litter in Wasdale.Especially around 3 peaks time. Lots of those plastic lights and wrappers.
Over the years I found quite a lot of kit on hills and mountains.
On the plus side:-
I’ve several sleeping mats - I’ve never needed to buy one.
I’ve several compasses - never bought one.
Torches, pen knives, maps, whistles, a belay jacket, a pair of Scarpa boots and a collapsable chair.
On the negative side:=
I left my Yashika T4 camera on an Spanish Andalusian summit and was unable to retrieve it due to bad weather and subsequent flight home…
Since I took up Radio & SOTA:-
Gains:-
An enormous rucksac = MTP PLCE Main Bergen 100 L | Military Kit on the way to G/NP-002. It was abandoned along with several other bits of kit, including a tent etc. It had been there for a long time, From the items left I’m guessing it was an abandoned Pennine Way attempt.
2 pairs of walking poles. Which I really don’t use much.
A dog on the way to St Sunday Crag in the Lakes (returned to owner and I still managed the activation - I wrote this up in an activation report a couple of years ago)
Losses:-
A pair of ski-to-rucksack ties left behind as I skied off GM/WS-017
Lost then Found:-
Left my very expensive Ziess binoculars behind after activating G/SB-001 luckily I’d only descended a 100ft or so before I noticed and managed to retrieve them.
A brand new LIPo battery I’d not yet used. Whilst activating G/NP-008 I was approached by a gentlemen who obviously wanted to talk. I didn’t want to speak as I was in the middle of a pile up. He was rather persistant so I sent "AS PSE " to the chasers.
The gent asked me if I’d lost a battery and showed me a battery he’d found a few meters away. I’d lost it whilst scouting for a sheltered operating position, he’d seen me using my radio and made a correct guess. Thank you kind sir, for both your persistence and kindness!!!
Lost: so far as I know, nothing. I feel sorry about. Should I try next time?
Found: A packed rucksack
On the way down from Piz Turba HB/GR-201 in August 2019 I found this rucksack beside the mountain trail but nobody around. I was shouting – nothing. I was looking behind some knolls around – nobody. Maybe someone was going there and got a problem. What to do? As I’m starting to call REGA (Swiss helicopter rescue, also coordinate other rescue) a lady with a (other) rucksack approached on the trail. She told me the story. 2 km away, after a break, she forgot here sunglasses there. After a while she realised the loss and her son, hiking with her, was heading back and leave his rucksack behind. Now, to save some time, she was transporting one rucksack a part of the way, put it down, going back and take the other. I was relieved. (On my further way I met this guy.)
Ludwig, I have left my rucksack like that lady a few times to go back for something I know I left behind - but I always try to hide so someone else doesn’t find it and … But I still worry!
David
Hmm, lets see.
Lost: Multi tool I used as a throw weight on a recent summit. Used it to deploy my EFHW but the plastic “S” clip unclipped. I watched as it sailed right off of the edge of the cliff.
Found: 1. FT2DR while following a certain fellow ham activator up a route to Raspberry Ridge (VE6/RA-086) last year.
2. Hiking pole on a well travelled summit.
Malen
VE6VID
I have lost a Dynamic compressor for my FT817 on DM/BW-055. Unfortunately not found again.
I also lost my cap once on the way back from DM/BW-103. When im arrived at the car I noticed this. I had to run back almost 1 kilometer again until I found it again.
Once I found a crossbow arrow. A loudspeaker box that was standing on a tree root (how did it get there I asked myself). Lots of caps for walking sticks. Now and then also times single gloves and so stuff. I usually have a garbage bag with me. So sometimes some garbage from the mountain comes away again.
73 Marcel DM3FAM
Lost so far:
Never found anything worth mentioning, sometimes just garbage, that I bring down, if possible.
Lost:
2m RG58 patch lead BNC male to BNC male
2m RG58 “magic” patch lead BNC female to SMA male with SMA to BNC male adapter (adapted all my antenna connectors to all my radios)
Pair of finger-less gloves
AA5TB style EFHW match box
Assorted plastic bulldog style mast clamps
5x tent pegs
Many pencils
Fishing pole top bung
SMA male to SMA male with 10cm of UT-141 used to adapt SMA coax cables to SMA handhelds
Lost and found:
Fishing pole top bung (lost permanently later)
GPS left at summit, had to re-ascend about 75m to pick it up off trig point!
Phone lost on walk out. Recovered next day using GPS trackback.
Found:
SIGG style water bottle in neoprene style carrying pack with belt clip
Lightweight walking pole
I have also nearly lost the will to live on some of Scotland’s more boggy and tussock grass covered summits.
A small list from me as I operate a strictly count it out / count it in routine on a hill, but sometimes things do go missing especially when there is a strong wind blowing! 
Lost:
Pole bung on GM/SS-205 Bogrie Hill – 29th December 2012
Small piece of plastic used as a seat on GM/SI-130 Sgarbh Breac – 13th May 2013 – blew away in the strong wind
Green 1.8 x 1.2m tarp on GM/SS-064 Tinto - 25th February 2014 – blew it away in a gale and I could not locate it; my tarps are now edged with fluorcent yellow tape!
Moonraker MRW-410S tri-band whip on GM/SS-276 Moncreiffe Hill – 2nd June 2015 – unable to find it despite quite a lengthy search; the replacement is marked up with fluorescent tape.
Found:
7AH SLAB on G/NP-029 Sharp Haw – 11th May 2009 – removed and disposed of.
Pair of pliers on GM/SS-106 Cauldcleuch Head - 8th July 2013 – still in use (nice quality, marked JCB).
Just remembered - I found a map wallet and OS map on the summit of GM/ES-037 Badandun Hill – 17th September 2020.
I’ve never really tracked down where to get a replacement top bung for one of my poles. If anyone fancies losing one then please do it where I can find it please
Mine are all attached with some cord now. Instead of losing them, they tangle in antennas when deploying on the summit 
Please see my note of 13th Jan in this post. The pole on the right has one of my caps made from self amalgamating tape. Somewhere on here I added detail on making one.
Regards
David
Alex
Have you tried an angling shop?? Those poles are the same as the ones which anglers use.
Lost a climbing carabiner on G/SP-015 The Cloud ages ago, was with a friend who was activating it and we used a climbing sling and a climbing nut to help secure a pole to the trig point.
End of the activation I packed up the sling an nut and put the carabiner on the trig point… got home and no Carabiner…
Not found anything on any Summits yet
Just left a really nice pair of trekking poles at the parking area of Strawberry Peak. I’m sure they were gone moments after I drove away.
Left a camera on the summit of Mt. Starr in the Sierra Nevada and went back the next day and recovered it.
Another country heard from
Eric
KG6MZS
Hey Guru,
Hope you and your family are OK!
I enjoy reading these post Sitting in the shack
many hours just looking at the screen for the
next spot, sometimes I play Sudoku Hi Hi !
Why would you " post withdrawn by author"
Don’t understand why people withdraw post!
73, Gary
Hi Gary,
We are all doing fine and away from Covid-19 so far. We are not yet in a strict lockdown but infection rates are again raising and measures may become stricter soon. Meanwhile, we are free to move within our Autonomous Comunity Navarra (EA2/NV) and this is very good to us with over 150 SOTA summits listed.
I’ll explain you why I’ve withdrawn my post: I created this thread with the title “…during SOTA activations”
My post was to say that I lost my SOTA baseball hat while hiking to Mt. Latxaga EA2/NV-065, which is true. But it turned out that we made a mistake and we ended on a different summit, so I didn’t actually make a SOTA activation and therefore considered my lost SOTA hat as lost in the mountains but not during a SOTA activation.
I hope you are doing fine too and you cope well enough with the lockdown.
73,
Guru
I use a wine bottle cork as a replacement bung. If you need a larger diameter use a champagne cork, or more likely a sparkling wine cork. Sand it gently to a good friction fit.
Prost!