Not only is that enormously rude, but it is outright thievery. If I caught someone doing that with my food (particularly a new carton of milk!!!), I'd report their behinds so fast it wouldn't be funny. It might make me unpopular, but I bet people would stop d*cking with my food. I think purloining food from housemates is a bit less of a problem, though. It's certainly to be avoided and not nice at all, but chances are they have plenty of other food in the house. Stealing someone food from work? It's quite likely that's their only food for the day. I mean, what if they're diabetic?!?! Not only could you just be ruining someone's day, but you could seriously injure someone's health.

Not at school and only on the very rare occasion at home where I don't realize I'm out of an ingredient and I know my roommates have tons of it (flour is a good example. My one roommate bakes a lot and has multiple 50 lb bags of the stuff in our garage freezer) I also feel less guilty about taking small bits of some of their ingredients because I know they've outright stolen my food and thrown out perfectly edible food before

I certainly would not open a new container. Ethics of stealing people's food aside, that's a very good way to get caught, or at least alert the person to the fact someone is stealing their food (my roommates did this to me just recently in fact. I opened my brand new container of cream cheese and the inner foil was still partially attached (which I never do with cream cheese) and some of it used. I knew for a fact I did not open the container, as I was finishing up an old one first))