I found this on the floor my shop. It's possible I dragged it in from the street but it looks purposeful. Very sharp tip on it, a knurled grip on the top and a small amount of threaded part on the pin to insert into something. I'm just not sure what it fell out of - any ideas?

Picture

Looks like the scriber tool that screws into the back of your combo square.

It ended up being exactly what you both said. I checked my square and found its home. I didn't even know the square had that!

Thanks for the help solving this mini-mystery.

FYI - Don't know if this is your case but I had the scribe come loose on my Starrett and can't remember how but there is a way to re-tighten the fit (google it?).

Some just press in and are held by a spring, most screw it though. I would expect that Starret to be srew-in.

Unfortunately, Starrett’s are all just a “snug” fit into a pressed in bushing. 

I have 3 Starrett squares and only 1 still has its scribe.