Agreed. I expect however that any easily available thermal sensing, especially on smartphones (see FLIR, etc), could be easily defeated with a strong adversary

Agreed. I expect however that any easily available thermal sensing, especially on smartphones (see FLIR, etc), could be easily defeated with a strong adversary doing good thermal management. As you say though, extra heat is a physical guarantee, so maybe a smarter technique exists to separate signal from noise in the thermal domain that I don't know of yet.