This sentence is from episode 21 of "Mirai Nikki". The context is that the protagonist is a bit surprised to find the other character sleeping on the second floor.
I know that んじゃ／のでは can be used in questions as an abbreviation of のではないか, but can just じゃ also be used as an abbreviation of じゃない with nouns? Is it widely used like this? I think I have listened to that quite a lot especially when the characters are speaking fast, but I'm not sure.