I was working on the wife's 2014 Honda Civic (battery drain issues) and noticed that right around the battery area, on a flat area of the engine block, there was an open hole that seems to go into the engine, maybe almost an inch in diameter and a rubber plug sitting next to it. The rubber plug has an "L" shaped piece of metal out the top, almost like a handle or such.
It is the same size as the hole so I assume it goes in the hole but anyone know what that is for? Do they pop out on their own?

 

The plug/cap for the transmission fluid fill hole. It should be plugged back.