I was inspecting my car up on the lift today with the transmission removed. There was a small sensor with a wire off it on the passenger side of the engine, towards the back of the engine next to a spark plug. What is it? I tried to take the plug off to see what the prong setup looked like and the plastic casing pretty much disindigrated.

 

Passenger side has two sensors towards the back. The knock sensor (in the block) and the fan temp switch/sensor (on the head). Probably the fan temp switch from your description.
Mine was really close to the exhaust manifold too. I was taking it off and it turned to dust.

S-D