I think the cops hear someone call them "sir" and they immediately think "lowlife scum trying to suck up." An absolutely neutral attitude is the best approach. Don't use titles at all, unless you know what their professional title is. If they're a State Trooper, you can call them "Trooper," a Sheriff's department officer is "Deputy," a plainclothesman is "Detective," and a city police officer is "Officer." Unless you get the rank wrong, like you were pulled over by a Sgt., or are being questioned by a Detective Sergeant. Federal officers are all "Agent." But if you don't know what their title and rank is, just don't use honorifics at all. Because that would make you start to smell like a weasel to these guys.