Dont think its a Volvo part.
I believe the sound for the parking sensors (factory fitted) is integrated into the cars audio front & rear speakers.
Do you have factory fitted parking sensors ?
Do you have or is there evidence of a tow bar being once fitted.
As sdp rightly says, it could be the "bleep" sounder for the indicators on a trailer / caravan ?
2016 Ice White V40 T3 R-Design Lux Nav, Xenium Pack, Comfort Pack, R-Design Rear Styling Kit, 18" Ixion Alloys, Rear Glass Tints, Active Xenon Headlights