For a number of years, I lived in a city. The neighborhood was good overall, but it had its problems. One was a house down the street. The occupants were known to be a good source for just about any stree drug, and they had loud parties regularly, with people spilling out on the street. We first tried talking to them--it didn't work. As soon as we started calling the police, we started seeing a pattern.
Within moments of the call (no matter who made it, no matter whether or not the occupants were told we were calling), all the young men would leave the house, through the back alley, and a number of women and children would come out onto the front step and front porch. When officers arrived, the women played dumb, claiming there was no party and that no one else was in the house (presumably true by the time they arrived).
I watched this happed repeatedly over a series of months before I finally got to talk to one of the officers about it. He told me that, from all the reports they had received, it was pretty clear that things were going on there, but that the occupants always "put on their show" whenever an officer was dispatched. He surmised, as had I, that they were using police scanners to monitor when officers were dispatched to their address. When the call came over the radio, their show started.
Yes, I know many of you may have concerns about secret police conversation, but I, for one, am tired of the bad guys getting away because they know the cops are coming.
[We left the neighborhood about 18 months later. We're still in touch wtih neighbors who report that they still see the little show, but not as often, and that they have seen them get busted once--they apparently had plainclothes officers in the neighborhood after hearing a tip about the party. That time, they snagged people in the alley after the call went in.]