After you filled the tank, did you run the water in any of the faucets? (e.g. galley or Head?).
The water system is pressurized with a pressure pump. If there is a leak in the pressurized portion (water pump to faucet), the water pressure will slowly drop. Once it drops below the threshold, the pressure pump will kick on and re-pressurize the system. Do you hear the pressure pump turn on for a second or two, then shut off every now and then? IF so there is a leak in the pressurized side.
If not, then there may be a leak in the drain. The drain is passive, so unless you are running hundreds of gallons, or the drain is disconnected, it will take a long time to build a sizable leak from the drain.
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