The connection between the siphon and the tank is completely kalked up in one of ours so there isn't a good seal.

Water drips into the bowl and every few minutes, the level is low enough for the tank to top-up again.
It's not enough to measure on the water meter but the inlet was kalked up too when we moved in and got stuck just as we went on holiday so no one noticed.

The water bill was more eye-watering than the anti-kalk!