There is a float (a floating ball) inside your Philips Humidifier's tank. When the water level becomes low, the float activates a water level switch and this will activate the indication.
This floating ball can become stuck and fail to activate the water level switch. When this happens, your humidifier will not display a low water indicator.
You can solve this by moving this ball with your hand.
If this does not solve the problem, please contact us.