Sorry guys, I like milk way too much I guess, I buy a gallon about every 4 days, and eat 1-2 bowls of cereal a day. Plus macaroni, and other foods that require milk.
The code thing is a joke, obviously this wouldn't be much of a topic if I just said "My roommates keep stealing my milk and I'm going to confront them right now."
My favorite idea is taking the jug with me into the bathroom. If they saw me do it, they seriously wouldn't even ask. They would just not drink anymore. I'll probably try the food dye first though. Just to be different.