There is a scientific principal (don't remember the name) that says that, when studying the unknown, the simplest explanation (that explains everything is best). Hence, the earth revolving around the sun makes more sense than the weird complicated movements you would need for everything to rotate around the earth.
It makes far more sense that either you, your roommate, one of your (or your roomate) friends is taking this food. The food that is missing is mostly junk food, the sort of stuff that is just grabbed and eaten (and forgotten about). You don't seem to lose whole turkeys, just some popcorn and Oreo's..
Really, how much money are you losing here? Is it even worth catching the person? Honestly, what are you going to do? If you catch the person, are you going to demand that they give you a buck (you can't prove they took eveything else). When they refuse, are you going to sue them?
I wouldn't mind hearing how this ends, but 99% of the time, it's going to be an obvious person. The idea that it's the maintence person (or some raccoon sneaking in) would be a great ending, but highly unlikely.