Newbie here. I have a 2012 Impreza 2.0 premium and was changing the spark plugs today. When I was done with the driver side, I noticed this line/cord that was coming from the dash area just to the left of the brake fluid reservoir. It’s a very thin almost plastic type wire that I see runs towards the battery. The end of the line fell off to wherever it is supposed to be connected. Can someone help me out and tell me what this line is for and where the end is supposed to run and connect to? Thank you very much for your time and help.

 

The way it passes through a blanking grommet would suggest it's not OEM and has been added by a previous owner.
If it was supposed to go to the battery then best option would be to trace it from the other side of the bulkhead in the drivers footwell and see what it's connected to.