Update:
Thanks for all of the suggestions above about checking the pump, etc. We had a pump problem several years ago, so we had checked for problems. But we went back and checked everything suggested above. No luck!
Similar to a service we once used in America, and based on comments from Pitak above, I found a plumber in Bangkok who uses a sounding device to find leaks. He said he would come to Pattaya and if he found the leak he would charge X Baht for finding/fixing it, but if he failed to find the leak there would be no charge for driving down. Set up an appointment for three days later. He drove down from BKK on Sunday morning and within a half hour found the leak under the floor in one of the downstairs bathrooms. He jackhammered the tile and cement, replaced the leaking pipe, tested thoroughly replaced the bathroom floor tile with one we had saved from the original construction years ago, re-grouted and left the area spotless. He was done within 2 hours of arriving and charged the original quoted price.
The guy and his assistant were great, and using the acoustic device to locate the leak probably saved other tiles and digging holes.
Again, thanks for the suggestions. Last time it was the pump as suspected. This time it was an under-floor pipe!