Here's my cracked idea for the day. I'm hoping someone here can lend a
clue.

I'd like to set up an array of audio transmitters that cover my
property, which I can use to monitor the activity of certain vermin.
They dig, and it makes noise. It just occurred to me that I might use
an off-the-shelf scanner to monitor all of the transmitters. Is there
any radio band where I could legally broadcast across, say, two dozen
frequencies? And are there off-the-shelf scanners that would scan it?