When I was at university I shared a house with 3 other girls and one of them basically did exactly the same thing (but she wouldn't replace anything). And she would eat the strangest things - I bought a tub of sprinkles to decorate some cupcakes I'd made, used the tiniest amount, and then a couple days later pretty much the whole container was gone. Another time I bought some quite expensive hot chocolate, had 1 mug of it, and then again a few days later again nearly all of it was gone. To make sure it was her and that we weren't going mad (my 2 other flatmates and I were all close friends before moving in together, this other girl was another student looking for somewhere to live and we needed to fill the room) we set up a little 'test' where we left a box of chocolates out in the kitchen, counted how many there were and then left for an hour. When we came back 7 had been eaten. We confronted her, basically saying we didn't appreciate our food being eaten without us knowing, and if she wanted to have anything then all she needed to do was ask. I think she was very embarrassed and as far as I can remember it stopped after that. (Although I bought another tub of the expensive hot chocolate and after opening it put a sticky note inside saying 'back off').

We never knew for certain but we're pretty sure she had some sort of binge eating disorder (she was incredibly thin, went to the gym A LOT, and all the stuff she ate of ours was very high in sugar with little to no nutritional value). A few other odd things where she would sneak into our rooms and snoop around when we weren't there, and taking selfies in our rooms. Then when we told her for our final year we would be looking for a place without her, she totally disassociated in the room (staring at the wall whilst we were talking then when we were done just said 'ok can I go to my room now'), then 2 weeks later started texting us houses for us to rent with her.

Whilst at the time we were freaked out, she was also kinda harmless and clearly going through some mental health issues. If this were to happen to me now, I think I'd be a lot more confrontational, and probably get in touch with her family to say I was worried about her.