Fill the car with water/coolant.
Place newspaper under the car to cover the engine bay.
Run the engine until up to temperature and the thermostat opens.
Hold it at med revs and look for water leaks on the new paper.
Jiggle the coolant hoses where they join the engine for leaks in cracks/creases in the rubber.
Listen for hisses.
I had a "mystery" leak last month. As you say, no obvious fault. Done the above, turned out to be a fine split in the pipe that goes from the block to the matrix pressure valve. Only found it by jiggling the hoses. Obviously not visable on a cold engine, as the leak would only appear once the pipe split expanded once hot.
