Here is an idea for using an entry sensor in the bathroom:
1) Place Entry sensor in small zip lock bag. (The half size zip lock bag that is about 2" tall)
2) Place entry sensor on bottom of the window sill. Do not use 3M command strips or screws, it can just rest on the sill.
3) The magnet can be applied to the moveable window frame and does not need to be in a bag, install how you normally would.