Sorry Canberra, it was "SONY" as in Sony_Man" not Sonny -lol
First time I heard an RMP referred to as a "Red Arse" was on a Brigade EX on Sennybridge ,when they were doing route marking. My driver was banging on remorseless about "f'ing redarses" just cos they got a f'ing stripe for turning up , etc etc etc", and he knew umpteen better routes to get to the RV, and didn't anyone ever teach these f'ing redarses how to read a map, you get the picture.
I've since heard it used to describe RMP's but perhaps it's one of those words not in general usage to describe the Bobby end of the Army?