I was curious about how forceful you saw pursuit of criminals in Sigil. My players got into an altercation with the Harmonium over the "Indep" slaying scandal.
One of the Hardhats was killed and I'm RPGing a pretty massive dragnet ala most Cop-Killers in the Real World.
However, I'm curious about how effective and important you run THE LAW in Sigil.
Don't forget, cutters, that the Hardheads have access to divination magic. With divination, scrying, commune, and discern location, they should be able to find someone if they want to badly enough. Of course, they're not likely to waste such powerful magic on petty thieves, and a real cross-trading blood would have protections against it. However, if a 2nd-level CE rogue managed to kill a beloved Harmonium officer, you can bet they'd pull out all the stops to find him — and probably kill him "while resisting arrest."
It's easy to stay ahead of the law in Sigil, you just need to know the right places and right people, and it will be difficult for the hardheads or mercykillers to get you. Though they'll be very persistent and in a lot of force, at least for a while depending on the magnitude, until something else bigger comes along, as is the nature of Sigil, and you're forgotten by most of them.