Its a bungalow, so all rooms are adjacent.

Bathroom -- I always open the window and wipe down tiles after a bath.
Kitchen -- I open the door and/or window if there is any steam.

All internal doors are closed at night. So I think it must be exhaled
water vapour.
Radiator is under bedroom window, but CH is switched off an hour before
bedtime.

Looks like drilling into the window is not recommended, then....

Bruce