I'd make absolutely sure that's the pin before tearing down the engine. I don't have one out close by to verify, but it is impossible for that pin to 'jump' out, unless the trough it runs in has broken out and if that's the case, you'd have a lot more metal in the sump. As Jeff said, it might be the clutch pin, but then you'd not have the clutch operating if it was missing. If it is the tensioner pin (and it sure looks like it), perhaps someone thought it was installed properly, but it popped out during assembly and has been laying in the sump ever since. If so, the bike has run since then without it. So if it is for sure the pin for the tensioner, I'd be tearing it down to replace. You can do it without removing the engine. This is a good reason to check the sump frequently after an engine rebuild - lots of critical stuff has been found there that had been installed incorrectly during a rebuild.