W124 OM602 300D 2.5T

Driving home from work the car began behaving very erratically, intermittent P/S problems, all the dash lights blinking, temp going up...

Pulled over, found the serpentine belt was shredding itself and had come off of the fan pulley. Got it back on, drove home, temperatures seem fine and no strange coolant smell indicating a head gasket failure...

BUT, when I had the car pulled over to fix it, it drained some coolant (300 mL?) from right behind the front passenger's side wheel. Looking under the hood, I can't find any source for the leak. The coolant expansion tank is visibly drained.

Any ideas?

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1992 300D 2.5T 249k - Parked with a bad transmission
1981 300SD 142k - Daily driver