Water loss that occurs while the pump is running and stops when it is off points to the pressure side of the plumbing: the return lines from the equipment pad back to the pool, and the fittings where they enter the pool wall. Under pressure, water is forced out through any break. When the pump is off, the same break stays quiet.
Air in the pump basket or bubbles from the returns can be a related sign, though that usually means the suction side is drawing air. Either way, the next step is to isolate the lines and pressure-test them one at a time instead of guessing where in the yard the leak might be.
Still not sure it’s a leak?
Describe what your pool is doing, and we will tell you what it may be and what a visit would involve. If the water level is dropping fast, call us right away.