I'm certain that it has only come from the guitar as when not in use it is kept in it's case which is kept closed. It's some kind of soft rubber it's half a cylinder in shape and 7mm long.
Don't know what a piece of rubber would be doing in a acoustic but is it important or what's it's function?

It's been rattling round in there since I got the guitar the only other thing I can think of is it's something that has got in there when it was at the store.