The police do use the normal airband, it is 'secret' at my local field in that it is not published, but your normal Com box will activate it if keyed on the right frequency.
There are 720 or so frequencies in the Aeronautical VHF band (assuming 25 kHz spacing). Five clicks = five seconds, then try the next frequency. So trying all frequencies takes 3600 seconds, give or take. That's one hour at most to find the right frequency. Although you might want to share the workload between a few pilots or you'll end up with a seriously sore finger.
But my guess would be that their secret frequency would somehow be "one-off" from any of the frequencies they normally use, for ease of remembering and selecting. That would reduce the number of frequencies to try to maybe 24 or so.
(A hackers mind is a joy forever...
)