Interesting topic Not more than speculation here, just an idea - would be thermal cameras that would capture size and shape of subject. By the way kinect is also could be used for this purpose, and maybe even more easily available. Basically that would allow to easily count persons in the rooms, even in the bed - if you have decided just not to get up for some reason Not sure if kineck could detect motionless person under blanket.
Another interesting add-on to the solution searching: Xandem alarm system. They claim that can detect and track more than one radio waves interfering object (subject). But they track changes in rf "net", so if someone just sleeps like a baby - not an option.
And if familiar with machine learning and video processing - I think possibilities becomes very foggy, but endless - some serious data processing and by shape, size etc. you can start recognising not that "if" someone in the room, but "who".