I found it in the park. It is made of plastic, circular, about 1 inch in diameter and flattened from front to back. It has a broken metal attachment which looks as if it is designed to hang from a chain as a pendant. It has an electrical "off" symbol stamped into it.

Inside is a coin battery fused onto a tiny circular circuit board with a plastic projection in the centre.

There is an exposed line of metal all around between the two halves of the plastic case, which might be intended to work as a wifi aerial.

It might be a key fob for someone's car. Sounds like it was dropped and crushed.

That's probably what it is. Someone is going to have a problem when he gets back to his vehicle!

At least in the US key fobs have several buttons for trunk open, alarm as well as lock/unlock but not on/off. Could be a remote start but also could be some other remote unrelated to automobiles. Posting a picture might help others...

Maybe a picture could help?

EDIT: michaelk's beaten me...

Press the button. See if anything turns off.

Any label or printing on the PCB?

My brothers and sisters would have had a field day and decide that OFF means to shut someone off, a.k.a. shut them up. Then point and click at a family member intending to shut them up 90% of the time. Accompanied by giggling on many others' parts, thus achieving their intentions. {sigh} big famil[CLICK!]

Without a picture it's difficult to place it but couldit be BlueTooth tag of some kind lie a "Tile" or similar?

A picture would be ideal but unfortunately I don't have a webcam. I'll see if I can get it on my laptop cam.

Update: I did manage to snap it but the image is too dark to see anything. I just don't know how to use webcams! And I'm currently on LFS where I don't have access to an image editor. Tomorrow I'll use the gimp in Slackware to try and brighten the image up and then I'll post it.

I've identified it! The dog walkers of Northwick Park had lunch together today and I showed it to the others. They recognised it at once. It's a flasher for a dog collar. That little glass tower on the other side of the motherboard is a discharge lamp and the plastic case is translucent.

You attach the gizmo to the collar and switch it on, and it flashes regularly once a second so that you can let the dog run free even after dark and still know where he is. But the battery is spent. When it fell off, it must have been switched on and it drained the battery. There's no point in replacing it given that the fastener is kaput. Still, my curiosity has been satisfied.

Interesting, thanks for solving the mystery. I've seen all kinds of lighted collar and harnesses now that LEDs are dirt cheap. Those are for visibility when walking in town. Even if the dog stays on the leash and on the sidewalk it is still good to stand out in the dark for the traffic.

On the high end there are collars with GPS and mobile phone service. You can track the hunting dog via smartphone when it's too far away to hear and call it to listen in on what kind of bark it is making, since some bark differently for different types of prey. I didn't find out if you could talk to the dog over the connection to call it back.

if sounds like something that may have fell out of a UFO, did you take any pictures of it????

Though OP's thing wasn't from a car key fob, I can say from experience that for some vehicles at least the remote transmitter is not essential. The transmitter bit for my car key is loose and fell out, the key still worked in the lock and the ignition.

/tangent

Glad to know that this case can be marked as [SOLVED]

No it can't be. It can only be marked as SOLVED in the technical forums, not the non-technical General forum it can't be.

I have upped the brightness and contrast on the picture I took and am attaching it. But it's not very informative. It just looks like a tiny yo-yo.

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