Fuel tank vent.

@Ironracer, the sender is under fuel at times. There is a chance with a very full tank and a hot day that a syphon could start untill the fuel level drops.

The factory vent on older models is on the highest point of the filler neck, it then goes up an inch or two then follows down the filler neck till it ends in the rear frame rail.

The tank would have to be fuller than the fuel cap or the car leaning to the left side significantly to have a syphon start.