CSAB id definately say he was doing the dodgy. I only remember the times i snuck out to meet dp, about 2 years ago, and also i ment up with friends when i was like 14 onwards. I must say i wasnt smart enough to do the whole clothes and pillows to make it look like i was home. But it def sounds like he snuck out hun. And the girls have a good point about the mobile thing.
Maybe if you got red paper and made 'tags' to wedge into the window, so that if it were to open, they would fly out and you would know it was opened?? Being winter and all i think it would be unlikely for a person to open the window, unless of course exiting from it. Or the bell thing, but that would alert him and be obvious to him, so he would jump into bed and pretend to be asleep and 'the wind' (or maybe the spirit) will have moved the bell or alarm.