Last evening, a car pulling out from parking hit me and my bike last night - no major injuries to me but the front wheel which I'd intended to replace in the near future, needs doing now. Anyway, I decided to give a bike a good wash in honour of the imminent new wheel.

I left it in my shed to dry and went out later to inspect its unusual clean state (I cycle in very filthy places) and I noticed a wee bolt lying by the rear wheel. I don't know if this bolt had always been lying about but, I have no idea if it came off the bike (maybe due to the collision) or it was waste from some other DIY shenanigans that goes on in my shed.

Any ideas from the attached photos?

Chainring bolt. Check your crankset for any that are missing.

Seems okay from this image - can it be from anywhere else on my triple chainset?

Check the inside of the granny ring. Could be one of those bolts.

I've just checked the tightness on the visible side - they're rock steady. Wouldn't they just spin freely if the far side were missing?

I believe your granny ring has its own separate bolts.

there are three places on your bike that use those types of Screws...
the big ring and middle ring... look ok in the pic..
the small inner ring... check it, let us know.
and finally, one might be used to attach the DERAILLEUR HANGER to the rear dropout... and those are usually the Shortest Screws of that type, unlike the one you just found on the floor.....

The outer and middle rings are held on together with nuts and bolts, the inner ring has bolts that screw directly into the back of the spider, sometimes with spacers.

The inner ring bolts are solid. I'm beginning to think that that bolt is renegade from a previous chainset replacement that my friend did for me -p robably ruled out from the woodpile that I lean the bicycle against LOL.

That's quite fun, but I like this one:

After pondering the possibility that the rogue bolt that had rolled out from the adjacent woodpile - possibly left behind by the boyo who replaced the old chainset - I took the bike out for a ride. All having gone smoothly, I now believe my theory was correct, although I'm clearly culpable for the wild goose chase.

Many thanks for your kind advice and attention.