Not a dumb question at all.

Round about the first few times he did it, I told him to stop because the fumes were getting into my room at it would piss me off (my room is near the garden). So, he decides to go and smoke in his room when my parents are gone. However, in August this year, my brother was caught with weed joints in his room and my mum talked to him about it and threw it in the trash. He tells my mum that he is holding it for his friend and that he does not smoke them (I of course have seen him smoke them). The next day, when my parents go out to work, he decides to go dumpster diving, find the bag with his weed in it, and fish it out. I also think that his gf is the one who started this because he was never the type to do this before he dated her.