Intel Research looks at tracking people you see all the time, but don't know, via Bluetooth: Mark Frauenfelder writes about Intel's Jabberwocky, which explores your relationship with other people, including what are called familiar strangers: people who you don't know, but recognize because you see them regularly. The first instantiation is a Bluetooth application that tracks the Bluetooth IDs it sees. The more frequently you see certain IDs, the more familiar the program marks them. When you're surrounded by actual strangers (or their strange Bluetooth devices), you'll see a different display than when you're near people you see often but don't know.
Once and if everyone is carrying Bluetooth devices, perhaps you could use this tool as a way to avoid detection. Private detectives will have to learn to turn off Bluetooth features to avoid showing up as known parties. Trying to avoid colleagues at work? As the colored squares grow in intensity, you zip out a fire door.