There are tons of fine points to argue here, but it's always better to be safe than sorry. Agreed?
That said, regardless of whether the smell is a legitimate PC, a determined cop or P.I. could quite simply sit outside until you spark something. Now, having likely SEEN you in action, he has PC...then a warrant.
If you're simply smoking though, and are doing it casually (as opposed to looking nervous/suspicious in your own home for fear of a raid, for example) I'd imagine you wouldn't exactly set off any 'alarms' within their department.
..........then again, I'm in Canada. It's lenient here.