Originally Posted by Indy'sexclusiveZx3
Looks like a vent tube
+1. Trace down the other end of that tube and see where it leads. I wouldn't be surprised if it is vented to atmosphere.
If you'd lost any gear oil, you would see remnants of it around the hole. It looks pretty clean to me. I would put the vent tube back in the hole, check the fluid level at the fill plug, and mention it to the dealer's service dept next time I'm in.