Grit - Can you smell the coolant in the exhaust at all? My Cummins F650 would similarly lose coolant mysteriously, but eventually I got to recognize the faint smell of rotting cotton candy (imagine throwing your kid's cotton candy in the wife's composter and coming back a day later), because it was crack in the EGR cooler (Cummins part, not the Ford one). It would always pass a pressure test because the crack would only open up at elevated operating temps. It would run like this for months before rupturing in a glorious smoke show.
I would think a radiator leak would leave some white residue around the pinhole.
