The water pipes are under the concrete. How long did they leave the pressure tester on? IMHO I think it should be 24hrs with no use of water. You could could shut your main off before you go to bed. Do not flush toilet during the night. In the AM turn on the water and go to that bathroom sink and run the cold water and hot water seperately. If you have "air" in the line you may have a leak under the concrete.
What about the "wax ring" between toilet base and the floor flange?
Another possible cause is "rain water" seeping in under the concrete. Do you have gutters to get the water away from the foundation?
