Mystery Bit Fallen Off

[compo]

... and I thought it was that nut holding the handlebars ....

.
Sort of daft response that appeals to my silly sense of humour

 

[Crackle]

I've still got an Allen bolt in a drawer that came off a bike. I dunno where and I don't know which one. I'm waiting for someone to crash, maybe me.

 

[e-rider]

As Cubist says, most likely a chainring bolt, but also often used to fix rear derailleur hangers to the frame. You say you have been fiddling with the gears so did you remove the hanger at any point?

+1 for this