Food going missing tends to happen all the time.... when I was living in a residence hall last year, that used to be among the biggest problems in any shared accommodation. My flat had lots of accidental ones because the fridge had only 4 shelves and the racks weren't evenly spaced so we had to share. It does happen that every now and then somebody will drink the wrong juice or milk.
My then gf's flat on the other hand, there were several horrible ones living there, food going missing used to be a daily occurrence.... 8 people shared 1 kitchen and there were 2 thieves among them. Canned food, cereals, bread and anything ready to-eat would go missing the moment you buy it. Biscuits, those would finish within an hour if you left it in one of the cupboards. Meats and vegetables.. those too can get stolen and to add insult to it, more than likely you will see some remainders of it cooked in one of your pans.
She was on a very tight budget, as were a few of her other flatmates, foods being stolen pretty much meant nothing to eat after that.
Hence, after the 3rd week, she kept all foods which can be dry stored in her room, other food stuff, we just bought enough for 1 meal as the store was just a short walk away.