Yep - one was found in my gearbox sump some weeks ago! 3.0 2000 MV6
The advice I got was if your box has no problems - leave it! Mine is as smooth as a babys bot!
I figure a strip down/rebuild would cost more than a replacement from a breakers - either way if it breaks I will be in the same position albeit inconvenienced with a possible recovery!
Am a little concerned but spose its a calculated risk
I have not held back on the gearbox since finding the thrust washer either!
I actually have a 2.5, and when I took the sumps off there was nothing - then I took the sumps off my doner 3.0 litre box and guess what I found
I want to see if there is a trend with this, and whether it is more apparant on the 3.0L's - Its odd, and it just seems to keep cropping up more and more on the forum.
Not only that but there are conflicting views, some people say it doesn't matter and like your box, carrys on regardless - other people say its the beginning of the end - however I haven't yet seen a thread where someone has had major autobox probs, stripped it and subsequently found these thrust washers, or at least bits thereof..