It appears to possibly be the fuel cap vent hose. The factory put a “T” fitting along the side of the fuel tank, in case the cap vent hose at the bottom of the bike ever becomes plugged. This way the tank would always be vented.
To check, follow the fuel cap vent hose from the cap, to the “T”, then all the way down to the bottom of the bike. The hose comes out just behind the kickstand next to the float bowl drain hose.