Take a single strand of hair and secure it between the latch bolt and the strike plate. Have it so you can see it(but only if you are looking for it) when you are looking at the door from outside your room. If the door opens, the strand of hair will be blown away so all you have to do is make sure the hair is still intact when you come back. Make sure your roommate doesn't see you trying to secure the hair though as it would be an awkward conversation and he would just replace the hair.
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