I did actually participate in the "strangest place" thread... but this one is different. No, seriously.
It was several years ago; most likely in 2004, but could've been '03 or '05 - don't quite remember.
I was on one of my weekly visits to what used to be my sport-workout place (yes, I know, no sense in having a workout only once a week... whatever), but by that point was probably already mostly various psychological talking (not that it matters to this story).
Well, whatever. So I went along the corridor, and near the cabinet I was going to was a couch. And on that couch, I saw something strangely coin-looking but completely the wrong size (for a coin I would know anyway).
So I just had to check what the heck there was randomly on that couch... and it was a brown American dollar.
Now that I know the proper terms, I can fairly confidently say that it was a 2000
Sacagawea dollar (don't remember the mintmark - if any - unfortunately).
But at that moment, I was just a very surprised 12 (or so) year old boy who was sure that American dollars only came in banknote form (well the
dollars anyway: I'm not sure if I knew that the smaller denominations were coins by then, but it's fairly likely that I indeed did) - and for that matter had no idea what the heck could make a coin turn so incredibly dark brown, and actually thought it was made that way (it was, of course, the tarnish Sacagaweas were such prone to).
...No, in case you're asking, I don't still have that dollar. It's currently in my father's set of various modern dollar coins, together with an
SBA, a loonie and a Mexican peso. However, I'm not sure for how long would my father's interest in it continue, and if it stops he's fairly certain to give it back to me; and then I'll have it again and could properly post about it

Oh, and this post probably won't be complete without mentioning the close second.
One morning in the winter, probably around 2009-ish, I went on my way to the metro, and saw a bunch of coins scattered on the road. Oh, of course I saw that sort of thing very often, but these particular coins were demonetized Soviet pieces from the 1980s.
I immediately sat down and picked up about a dozen of them (it was a huge bunch). However, it being in the winter, the lower layers of the bunch were somewhat encased in ice, and the coins being technically on the roadway (though right next to the sidewalk), I didn't want to stay picking them up too long either for fear of being hit by a car. So I took that dozen or so and went along on my way.
When I returned back to that place in the evening, there were no coins around anymore; I don't know to this day what did ultimately happen to them, nor where did they come from in the first place.
Alas, neither do I know whether I kept the coins I
did pick up (and if yes then where), as other than the circumstances of their acquirement they were actually fairly unmemorable. Sorry
