Hmmm... Back in the day when our rifle (and pistol!) club was also a Registered Firearms Dealer, there was an entire belt of .50 Observer/Tracer ammunition in the armoury. I didn't get a close look at it, so can't say if it was standard .50BMG or (rather more likely) a spotting round for something, but it definitely had the red+yellow tip.
(It was in the UK, so is long gone by now, and would definitely count as "prohibited ammunition" under the current regulations.)
I assume they picked it up at one of the auctions and later found that it wouldn't fit anything (quite apart from being unusable on just about any .uk range due to the fire hazard).
Chris.