I think that's the vent for the battery. The idea is to vent the fumes to the outside of the car.
The other end exits to the outside the body somewhere. Trace it back and see were it goes.
There should be a hole in the top cover of the battery that it plugs into.
Also....Check for any spilt acid around the bottom of the battery. Because that's not plugged into the battery top you may have lost some battery acid.
