I smell weed in the elevator of our non-smoking apartment building regularly, because someone has walked outside, smoked, and returned to their apartment at some time before I had to leave. I usually know before the elevator even reaches my floor. And you can't smoke at our entrance either, I've seen them walking back from a nearby park.

I've smelled weed on my own clothes and hair days after visiting people who smoke, when I never have. It's strong enough I've had friends comment I smell like weed in this scenario. These days whenever we visit them we immediately shower and do laundry as soon as we get home.

People were polite or slow to assume the weed smell came from the teacher. I know I would be. I ignore the smell of weed and regular smoking all the time. There's no amount of sneakiness sufficient to stop it clinging to your clothes and hair.