Coolant leak, but no idea from where...?!?

No, you are looking for a kit that contains radiator cap adapters and a hand pump with a gauge. Ask for a radiator pressure test kit at your local parts store. If you hook up your compressor to the radiator, you will be needing a new radiator. The leak you are describing sounds like the classic exhaust stud leak. The 2 exhaust studs at the ends of the head screw into the water jacket. If you install them without thread sealant the result is the leak you are describing. The fix is remove the studs and reinstall with sealant. Drain the radiator when you do it or you will have a big mess. Don't forget to do both sides while you're there.