Oops, sorry folks… totally forgot I’d posted this thread, so only just came back to it. Thanks for the responses. I just wanted to calibrate my response, really - see if I was being too lenient.
Backstory is that I’m not rabidly anti drugs (have done too many myself, in my time), so always took the approach of open discussion about the risk-benefit ratio of various drugs including alcohol. However… despite all my discussions about moderation being key, and the effect of cannabis on developing brains, last year she was given a joint by a friend (unbeknownst to me) and smoked the whole thing on her own. She had a horrible time and some lingering after effects, so I’d strongly advised her not to smoke again.
To all those judging me for clearing and cleaning her room… calm yourselves; it was an extremely rare occurrence. 
It’s been a pit of chaotic teenage horror in there for the last couple of years, untouched by me. I don’t think it’s at all good for her mental health, though, and as she’s been holding down a job and working hard at college (and struggles with executive function), I suggested renovating her room as my Christmas present to her. She was happy and relieved, because she felt it had become unmanageable in there.
After some consideration, I put the joint with the rest of her bits from that location, intending to mention it to her at some point. She actually asked about it the next day when she saw I’d cleared that area, and that opened a frank discussion. I’m hoping she’ll now throw it away.