Non-visual coolant leak

Since early spring, we have been having to add coolant. At first every two hundred and fifty miles, then every one hundred and fifty miles, then down to every eighty or so. We just changed the water pump, with no change in the leakage. We check for leaks, nothing. Occasionally it appears to dump a large amount, but we believe this is from the air conditioning, no orange color. We did drain the oil and a cup or so of coolant came out first. Oil is not milky, the car runs good, idles good, acceleration is good. Any suggestions? Also, the car has never ran hot or overheated.