Have you ever found, or encountered an object and never figured out what it was?

I believe the title says most of it. It must be an object. A thing. Something you may have found or come across, and you didn’t know what it was. While my questions suggests that you may have never discovered what it was, if you have discovered it eventually, this does count.

But here’s an example.
Now I didn’t find this thing, but someone I know did, and I saw it. Back when I was playing Dungeons&Dragons years ago, one player showed up to the game with a strange object. It was this ball made out of metal, about twice as big as my fist. All silver, kind of shiny, with a round indentation on top, or what I assumed to be the top. Looked like you could open it at first, but you couldn’t. Nothing else was on this. The guy said he found it in a park, and we puzzled over it. Someone suggested that it might be one of those balls that people throw at Olympics and stuff, and whoever throws it the furthest wins. But then someone else said that if that’s what it was, the weight of the ball would be on said ball. I don’t know nothing about that sport, perhaps true, perhaps not.

So of course we joked about it all night, saying it might be from some aliens, or that it was filled with uranium and we’d all be dead pretty soon. The guy who found it said he would bring it to the university and get it appraised. D&D joke sorry, but he really did say this.
I was really curious to know what it was, but there was never any follow up on this. Don’t know if the dude got it ’‘appraised’’ or not. Nobody ever asked. To this day, still don’t know what the hell this shiny silver metal ball was.

So that’s my example. Have you any stories about strange objects you found, and that you never figured out what they were? As I say, if you know by now, the story is still valid.