Flat land Dynamic Soaring?
Ollie, I've seen this quite a few times when flying free flight. On VERY dew laden and soggy mornings and especially when foggy but without rain. The air near the ground just seems to be all upwards to about 100 feet or so (about your telephone pole height) and then tops out. Coupe d'Hiver's and P30's sent up in this stuff just hit that 100 foot mark and loiter at pretty much the same height or only loose 20 feet or so before the DT cuts in at 3 minutes. This sort of behaviour is very repeatable here in the Pacific North"wet" and is hardly an isolated incident. I also had a Gentle Lady morning where I got a 20 minute flight over a plowed field in dank cloudy DAMP conditions. Nothing I did in this case would get it higher but if I left the zone and came back it would oh so slowly work it's way back up to the same height.
I think it's the dew forming a very moisture heavy layer close to the ground. Something in that layer is lifting the models up but I have NO idea what it is since you can't feel a darn thing moving. Perhaps the mass of the moisure itself being absorbed into the upper air forms an upwelling of dense water vapour laden air that disapates as it gets higher? And that is what mimicks thermal lift? Just a guess.
I DO know that I don't trust anyone that says their model gets such and such a time in still early morning air. It's all false given the way the free flights stay up in dew and fog.