Should I tell my neighbors that their teen is home during the day — and smoking pot?

I live in a small apartment building where we all know each other and each other’s children. I frequently work from home, and it’s come to my attention that one of the teenaged children who lives here is at home during the day (on school days), smoking marijuana — which I can smell in the hallway — when both his parents are at work.

What do I do? Assume the parents know? Talk to the kid? Tell the parents? I want to be neighborly, but I also don’t want to be the silent accomplice of a kid who may be a truant.

— Joe Next Door