To answer your question, before the days of trunktracking scanners there was a board which could be installed into certain scanners, which would detect those beeps and return the scanner to (conventional) scanning. I believe it was called GE-whiz, though I may not be remembering that right. You might try searching around usenet archives of alt.radio.scanner and rec.radio.scanner for more info.
I supposedly heard one of these after-market gadgets in use at a local wrecker's shop, but I didn't get to actually see it and my then-weeks-old Pro-92 was doing a lot better than it.
My local EDACS system used to have buzzing stepping through the vacant voice channels, assumed to be there to defeat scanners. Initially it was nearly continuous on one channel or another, pausing briefly only to change channels every five seconds. Later there would be a bit of a break before it resumed on the next channel. I recorded
a WAV of it a long time ago; I still hear it in a different context, from time to time when someone doesn't quite properly connect. Back then it was way too regular to have been this, if the control channel was 1 then it would go 2-3-4-5-2-3-4-5-2-3-4-5 for hours/days/weeks/months on end, to be interrupted only by actual comms on the system. I haven't scanned the system conventionally in years so I don't know if it is still there.
If there is another reason for that buzzing, I would love to know it.
Jim
EDIT: found a link:
GE/Ericson trunking scanners? (rec.radio.scanner) Looks like this one came from Scannermaster. As others have written in this thread, and despite what is written in that link, please don't mistake those beeps as being purely to defeat scanners, although that side effect may have been quietly marketed to prospective system owners/operators.