2007 F150 4.6L

I'm having trouble with an antifreeze leak. Wondering if anyone knows what this cap is called.










I've been working on tracking this leak for sometime now. I filled the resevoir, started the truck, watched the level raise then drop as it pressurized. Didnt see an immediate leak or drip anywhere. I just know that the fire wall on the passenger side is damp with antifreeze. The hoses going into the heater core are bone dry the main saturation is in the middle behind the motor and on the lower passenger side.