Also have a read of this:
Uniden Close Call - The RadioReference Wiki
"The feature is designed to catch signals that are "Close" (hence, the name). While you might not get a hit often, it should be the case that any hit you do get is from a relatively nearby transmitter, so could be of enhanced interest to you.
For the Close Call feature to detect a signal, that signal must be about 15-18dB stronger than the RF noise floor in the band being checked. This means that there is no real way to predict the range within which signals will be caught. The factors that influence range, however are:
RF Noise Floor in the band (lower noise floor increases range)
RF Power Output of the Transmitter (higher power increases range)
Antenna being used to receive (too complicated to predict, but it does have an effect)(by acting as a tuned system, it can preselect within the required band. Other out of band signals will be attenuated) )
Note that "RF Noise Floor" in this context includes signals from any other transmitters operating in that band. So, at busy airports, getting a hit might be difficult (lots of VHF signal sources all the time, as well as a few continuous VHF broadcast sources). In general, a 1W portable will be captured up to around 100' (30m). 4W portable around 250' (75m), 25W mobile up to a half mile (805m) or more, and 100W base stations up to several miles/km. Other signals will affect Close Call range due to the nature of nearfield reception."