I forgot some things:

1) Cars that sit a lot or sit a long time can develop corrosion on wiring pins and terminals, these, again, can cause erratic readings which can fool the ECU.
2) Cars that sit can suffer from chewed wiring if rodents have access to the car.
3) If the wire in pin 1 has unstable voltage on it, check the other two pins to be sure the 5 volt lead is SOLIDLY at 5 volts, if not, you must trace back to the ECU to see what is causing it to be unstable.
If the ground pin has more than maybe .05 volts on it, you must follow that wire to the ground connection to see where the resistance is.

You could have a system leak, but you could EASILY have a wiring/sensor issue that makes the ECU THINK you have a leak. Don't go too far in "fixing my leak" until you KNOW that the 'leak' is real.