Don't steal my milk!

Grrr what a grumpy morning this is! I get into work really early, about 6:45 so to avoid spending time packing a breakfast and lunch I keep food at work.
So I go to have my morning bran flakes and someone has finished off my milk!! Seriously if you didn't put it in there why would you think it's ok to take? And it's not that they used it, it's that they FINISHED it and now I don't have breakfast!!!!!