Ordinary bluetooth 25-30 feet in ordinary conditions, up to 100' or more "in the open."

Using a high-gain directional antenna, a Bluetooth "base station" can detect and identify an ordinary consumer device (like a smartphone or earbuds) at distances ranging from 175 meters to approximately 450 meters (1,476 feet) under optimal line-of-sight conditions.

So, if you've got a "secret meeting point" you could conceivably setup checkpoints on all the access roads and detect police travelling along those roads within 200-300 meters OF THE DETECTOR - the detectors can be setup miles from the meeting point...