OK, new pathfinder owner here. Got an 05 SE 2wd model with 200k. Its had most everything replaced on it including trans and radiator. While combing through the engine, I came across this broken cap or valve on the passenger side near firewall and heater hoses and I do not know what its supposed to be or what it used to have on it. Any help would be much appreciated!!

That appears to be the water valve for the rear heater core. I am looking at that broken piece and can not believe that is exactly were mine broke a few months back while parked. Mine is the XE model therefore does not have rear heater, so it was the valve for the front heater. The SE must have an additional vale. Is your rear heater working?
Now my story: while I was parked after coming back from lunch I popped the hood and had noticed a few drops of coolant right under that valve and as soon as I wiggled it, it exploded on me and bursted all the hot coolant , luckly I was not burned or scald. Nissan has a problem with those water valves and I believe it is because of the heat generated above the exhaust manifold that with time it weakens and becomes brittle until one day BOOM!. people beware. When I took it to Nissan the first thing they said is OH YEAH WEVE SEEN THAT BEFORE!.

They call it a "heater hose assembly." Refer part code "92414" at the part diagram linked below:

http://www.nissanpartsdeal.com/parts-li ... b6%3dSE%29

Looks like you need Nissan P/N: 92408-ZL90B

OK, sigh of relief. After further examination, it had previously already been bypassed and fixed. They just left the tube attached and didn't bother removing it when they put the new line in.