Reply #20: Oh, give me a break. This is NOT the equivalent of smelling pot at parties.

[Berry Cool]

This is hardly a Nazi-esque situation. The OP is talking about a workplace, for crying out loud.

I'd say, tell the boss about it, but say you had no way of knowing who it was and were not going to snoop and find out. That way, as some others have said, the most that can be done is that the management can remind everyone that drug use on the job is not acceptable. The person who did it can think "uh-oh, almost got caught, never gonna do that again at work" and behave more wisely from then on. If that person chooses not to do so and gets caught later by a more watchful management, it's not your fault.