1999 Caribou Quarter in Circulation?

Id keep it.
Stick it in a 2x2, and hang on to it.I'm assuming your using a Whitman or Dansco album? Your right tho, trend values and the Charlton are only providing values of grade MS64 as indeed they were never released during normal circulation. You may want to sell it as well. Im sure there are lots of collectors like me out there that try to collect an example of every variety of every year.
Who knows whats in store for value down the road, if you hang on to it for 50 yrs..

I myself found a 1999P 25ct/quarter in circulation.Its not the best of shape with rolling machine marks but never the less.I put it in my collection as circulated.The 1999P set were the first P coins made.

Could it be also possible the RCM tossed their excess of struck 99 quarters that were over the number of sets required, into circulation, rather than melt them down?

That's an interesting story but I would be very skeptical. The mint usually produces test tokens of the same composition, weight and size. I have several in my collection. I think you are right that the mint decided to cash in on the interest and there are about 7 pages of them listed in Charlton.I was in the Vending industry and could have requested test tokens, sometimes I'm just not to sharp.I have one, 99 caribou, as well but I bought it as a single. I heard a good argument for having them as they complete a caribou set for those that like that series, I do.