I still don't know where any of the sensor connectors are supposed to be mounted, but the mystery connector was for the preheater off the intake pipe. It took a crazy amount of time to find the single component with green/orange and black wires in the wiring diagram, and then figure out what exactly a "PTC resistor preheating" was. Good times. I don't think I ever would have found/remembered that little connector in such a weird, obscured place. Thank $deity for wiring diagrams posted to the internet.