In the US and Canada, they can search if they smell or suspect anything illegal. It's pretty subjective. But they likely won't search unless they think there's a lot of pot in there, because it's a lot of paperwork involved.
I'm thinking it's different for different States then. Where I've lived in the US the cops can't even legally search based on a smell or an anonymous tip or anything... unless there is another primary cause for the search.
I had neighbors in college that were selling what turned out to be heroin and my roomate called the cops on them. They explained that that couldn't do anything unless the cop caught them in the act or there was some sort of violent event, etc. because they can't search based on hearsay. Luckily they had an unmarked police car patrol around the apartment complex for a while and caught them doing a deal in the parking lot.
I definitely don't know about the individual laws. But as for the cop being able to smell it (not for the hearsay evidence), it may be legal to search the residence, but it doesn't mean he/she is going to. Likely they would just find some high dude sitting on his couch watching a chick flick and eating chips. So it's not worth it.
But, TMonster, maybe if you called the police station (not 911) and asked them to talk to your neighbours for you, they could give them a good scare?