Bear in mind that actual range is NOT going to be even a quarter mile.
On low powered stuff like drive in windows coms, it might be 25 feet. For someone's 1 watt handheld, maybe 50 feet. A 50 watt mobile unit passing by (rare that they're that powerfull, though), maybe 150 feet.
That FM Broadcast station over there running 10KW? Your 1/4 mile, or maybe half mile.
These units work by detecting things stronger by a significant amount than the 'radio floor', the general level of signal present from all the users around. In a big city, that 'floor' is pretty darn high, and something has to be very close to pick out.
Out in Plaster City, California, rural, maybe it'd work better... though there's less to hear, o'course.
If you actually want to know 'who transmits from near me' other than wandering mobile units, the FCC Report site will happily provide you with a list of every transmitter located within a given distance down to 1 km.
They'll work fine for discovering your OWN system's frequencies, though you'll still need to use some intelligence to figure out the full information. They'll do you no good if you're on one of the 'unmonitorable' trunk systems like Pro-Sky, for instance.