You're right, of course. I guess it's possible that they're getting in through some hole in the wall, that just happens to be located beside the piano. To be honest, it's unlikely that these new moths are the same infestation as the old moths that have been entirely absent for 15 years - that seems fairly unlikely, really - so maybe the source IS somewhere in in the walls. Maybe this weekend we'll have a stiff drink, and do some furniture moving...