I had a similar sounding problem with my 1.8 K series coupe a few years ago There was never any puddles under the car, but I was loosing coolant somewhere. In the end I found the thermostat o-rings were the culprit, so check the back of the engine behind the water pump.
Weird how it's exactly the same part (CDU3858) that gives me grief on my ZT 190 these days.