Factory flaw it turns out - just for future reference to others with this issue, the separator box was mounted to the rear tub with the bottom forward corner rubbing on the seat belt bolt, and it rubbed all the way through opening a pinhole leak. Should be able to JB plastic weld it and eliminate the problem (while re-mounting the box with some washers etc so it no longer rubs). How annoying though right? A design flaw specific to ST 90s made with a rear bench seat.
Definitely SMOKE TEST was the best advice.