Thanks guys. I learned a ton from this thread. And have actually already had a chance to use it to some degree.
Pulled into my neighborhood the other day and there was an unknown vehicle and an unknown woman, teenage girl and very young boy milling around a vacant, forsale home. I casually made the loop an took down their plate number and got a good look at the people. Pulled into my driveway. From there I could see them to some extent and watch. Police were called, my information was given, and as we were talking to police, a cruiser pulled up to the people in question. (no the response wasn't that fast, there had been an alarm set off)
It didn't look like much came of it. Cruiser left and the car left. But it was nice to feel like the little situation was actually handled properly.
(no I didn't think a woman and two kids posed a threat, but they looked alot like someone scouting or playing lookout)