It's funny but I think it may be more than a few scum bags.
Organised bike crime is now such a huge problem most counties have set up specific divisions to try and deal with it. You will see lots of posts on here from a Bristol Policeman trying to get bike back to people and taking leads from us.
I guess all you can do is make sure the bikes are covered or you take them up onto the balcony when you have finished your ride and look them up there. But at night I sleep with mine in the tent.
Did you know you can get motion detection lights and alarms for tents now! May be worth a look at that.