I don't know about other places, but where I am (in the states) there's such a thing as internal possession for marijuana. (Not sure if it's just my state, tho.) So just because it's not on your person, doesn't mean you're still not in possession. Which is a bummer.
(This info is what I knew about 2-3 years ago, so it may not be accurate. Hell if I pay attention to anything anymore.
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