Ah, sorry, mistead that. That means they were doing police work then, I think. That's dangerous.
No...no, it doesn't.
Police work can increase the chances of catching an infiltrator in the act. It does not, so far as I can recall, change the chances of catching them with patrols—and even if it does, it's not
necessary to have patrols catch them. It's perfectly possible for any unitless noble, infiltrator or not, to be captured by patrols in a region with no troops at all.