While this clearly sucks I doubt they did it to upset you...some people are just more comfortable "sharing" for example there is the classic office work problem where food can disappear from the common fridge some people seem to actually think they can just eat what they want or have no ability to control it---I'm not sure how they think because I've never had anyone fess up to it but there is a wide range of personalities one of the reasons I tend not to enjoy roommates etc. I was in lab one year and we had gotten rid of the Styrofoam cups to save the environment and save $$$ for the department and we had a new summer student who was holding my personal mug and I asked her why she had my mug which was a gift from my sister and she was just like Oh I didn't think it belonged to anyone....seriously? I had it in a plastic baggie so it didn't collect dust and everything so it clearly wasn't abandoned but I guess she just really wanted some coffee or something it seemed insane to me that she wouldn't ask around if things belonged to people at the very minimum. People are so diverse and that is the nicest thing I have to say about that.