Hmmm. I'm a bit torn on this one. I'm a host, and my listing states no smoking. No smoking in an Airbnb will generally be interpreted the same way no smoking is interpreted in virtually every other situation worldwide - that is, no smoking indoors. This is how it works in virtually every hotel, every bar, every restaurant I've visited in each of the 70/80 countries I've visited.
Why would you be upset at anyone smoking outdoors? Unless they left a mess, which if that was the case it was actually a cleanliness issue.
I really think you need to be conscious that we are 'hosts', not wardens or enforcers. We host. We don't judge within reason. You should accept people are people - every one of us different. Yes our basic rules should be respected, but overzealous hosts give the rest of us a bad name.
Your situation is certainly more complex. You know your guest smoked in the garage This is technically against the rules, so you're in the right, though it also comes across as a (misguided) attempt by your guest to respect your no smoking rule. However you only know this as you spied on them with a webcam. That is a big no no, and possibly illegal. You cannot do that. And you should realise that is just creepy.
And where has it got you? You are now wound up, annoyed over something that "might have happened". What on earth are you hoping to achieve for yourself giving yourself anxiety over something you can't tell happened? I think most people have a no smoking rule so there isn't a smell of smoke (which you can't), not because they want to impose their will on others. I think you should reconsider if hosting is for you.
