The "main character as PI/Cop" thing is leaned on heavily in urban fantasy, just as it is in mystery/suspense novels and for the same reason. Murder is big stakes, and if your main is a PI/cop, it's easy to explain why they're involved with a murder book after book. Otherwise, you end up with your gardener-main-character involved in one murder after another and the reasons for the involvement become more and more contrived (found a body in the garden in book 1, found a body in someone else's garden in book 2, and in book 3 she found a body in the gardening supply store). You start thinking the cops should just tail the main character around, since she'll eventually lead them to yet another body. Plus, no reader sits there and wonders, "Why doesn't she just call the cops?" when your character IS a cop.
So if you don't go the PI/Cop route, you have to think of a main plot that has high enough stakes to catch the reader's interest but the cops don't know about it or won't investigate it.
Mercedes Lackey's Diana Tregarde was a romance novelist who had a "calling", I guess you could say, to investigate supernatural crimes, and in one of her three books she was called in by the police to help investigate a series of murders.
Tanya Huff's Vicky Nelson is a PI/ex-cop, but her spin-off series features Tony who's working his way up from production assistant on a bad TV show while he's learning about being a wizard.
Laurell K. Hamilton's Anita Blake started off as a professional necromancer but also had limited Federal authority as a vampire slayer and, last I read, was a Federal marshal.
Rob Thurman's characters Cal and Niko are ... well, now Cal's a bartender and I dunno what Niko's up to when he's not being a bad-ass.
Marc del Franco's Connor Grey is a druid who's lost most of his powers, but used to be a member of that universe's magic cops (basically). He's a PI now who works with both the mortal cops and the Guild from time to time.
Simon R. Greene's John Taylor is a PI in the Nightside.
Rachel Caine's Joanne Baldwin is a Weather Warden... sometimes. It's complicated. But she's one of a group of people who tries to see that the weather doesn't get too cataclysmic.
