Mogman,
I suspect it was added or like suggested, it's a road vehicle.
On an RTV I would think the linkage under the dash would be the location for the switch that turns the beeper on, so my first look would be there.
The only other place would be on the top of the transmission where the linkage is for the transmission. The off/on switch could be mounted there.
The rest is cables between the shift linkage and transmission and unless they have a special cable with built in switch it should not be there. Me thinking it's too expensive to manufacture a cable for a backup beeper. Simple on/off switch must be somewhere that is easy to mount.
Keep looking.
Oh, and if you find one, just turn the key on and press the switch to activate the beeper to help identify if you have the right switch.