She was in pretty bad shape when she showed up, if I recall correctly, and I think we're supposed to believe that the cameras are only on the door and outside so the only time they'd see her is if she left (which she didn't) or the one time she pushed herself against the wall to pretend to cry. There was one shot of a camera that might have been in the middle of the room but not sure where it was.

I didn't like that addition (it's not in the book) but so much of the ending is pulpy and silly that I (and I think most people) sort of just accepted it. But, yeah, it seems to create more issues than it solves.