Check the return line from the injectors to the tank.. No pressure in it but when the engine is running, a crack in the rubber line will leak.. On the QM, I think the return goes back to the secondary filter.. but I'd suspect that line and the fittings. I had that and it took a while to find it.. The hose was the original from 1985.
Otherwise any pressure fitting past the lift pump.. The banjo fittings at the lift pump, the secondary filter, and the HP pump are all suspects.. Use a clean tissue to touch the fittings when the engine is running and the one that wets the tissue is the culprit.