The meter test did not detect any movement.

There is a leak in the water tank itself or in the water supply pipe on the first floor behind the water tank.

If you can visually check for leaks in the water tank, check the water tank first. If it is difficult to visually check, check for leaks in the water supply pipes on the first floor first.

For a full-scale leak detection, we recommend that you contact a professional leak detection company for consultation.