Andy, You may or not have read in my report Curtains on Craiglich that “local” G and GM’s were hearing my 10m SSB transmissions, so that ties in with your findings.
Rumour has it that wspr is such a good weak signal mode that a piece of coax lying on the ground connected to a dummy load will put out a good enough signal.
I certainly had a 10m wspr transmission picked up in Antartica a couple of years back, and that was overnight.