My X5 keeps displaying 'Coolant Level Low.' There must be something going on.

About two weeks ago, the warning came on, and I brought the vehicle to service dept. The service tech said coolant level was really low, and it was refilled.

In about a week, the warning came on again. This time the service dept performed 'Pressure Test', but they could not find any leak. Coolant reservoir was refilled.

Five days after, the warning message was displayed again. Service department refilled the reservoir again with 1L(apprx. 1 quart) and booked it in for service.

The same frustrating message was displayed again last night in three days. It's very frustrating.

The service department says if the Pressure Test doesn't find any leak, there is no leak. Is it true? Is there some other leak that cannot be found by 'Pressure Test'?

Where are common leak points in X5s?

Thank you very much in advance for your inputs.