'01 LB7: Doing an injector replacement, killing time undoing everything to get to the valve covers.

On the left side of the engine, behind the upper radiator pipe support bracket, is a 2-tiered bracket for 2 ZIF connector heads. On the bottom side of this bracket, is a pressure sensor. I'm not seeing anywhere that this sensor plugs into- no port on the valve cover, no hoses that appear to be loose or missing- the rubber collar just seems to be sitting in mid air. What is this sensor for? This wouldn't be for atmospheric pressure would it? (It looks like a pressure switch, but to what?)

Any ideas?