My DM has my DD at my house one afternoon a week while I'm at work. Each time she comes she turns our thermostat right down while I'm out. DH doesn't always notice as when he gets in he puts DD to bed and then sits in the lounge which is on a different thermostat to the rest of the house. Each morning after she has been I get up and the house is freezing and then takes ages to heat up. It drives me nuts. I get home from work after midnight so go straight to bed and rarely notice.
AIBU in thinking that if you want to adjust the heating in someone else's house then you should put it back or mention it when you leave?