That is shocking! And next time I'd go into the hall in my PJ's.
I'd knock on all your neighbours doors, and ask them if they have seen anything suspicious, and tell them about the theft, the reactions will tell you a lot. If you think you've got the thief, tell a good sob story about how you can't afford this, don't have a car, can't carry the boxes etc. Seriously guilt trip them, and mention you can't believe anyone in the building would do this, it must be an intruder.
Ideally you'll have a busybody there that can keep an eye out, and you'll have an idea who the culprit is.