I did this once on Worcester Beacon WB-009 - I left the RG213 in the car. Strange, because I used to carry it by wearing it like a sash and how I missed carrying it I don’t know. Fortunately I had a couple of patch leads which let me get the antenna to all of 3 feet off the ground. I now use 5D-FB 9mm semi-rigid which fits inside the backpack.
I was going to suggest the obvious - make a checklist covering the kit, clothing, food, etc and print a copy off before each activation, then tick off the items - but of course that doesn’t prevent brain fade on arrival at the parking spot!
We should start a club - I did exactly the same on Hill of Garvock earlier this year. The frustration of knowing you have several leads sat in the car boot a mile away but insufficient time to go and fetch one has to be experienced to be believed!
It’s not that bad. I forgot my Goretex jacket once which meant I couldn’t go and do the walk as it was February and raining somewhat. At least you got to enjoy the walk even if you couldn’t do the kind of radio you wanted to. Also, you got to see one of the oldest trig points in the UK. Can’t be that bad!