It's possible that the water pressure variation from the well-pump or municipality caused the system to go into production mode. Or if you have an icemaker in the fridge, it might have been making some ice and using the water.
Otherwise, it might be an indicator of a possible leak of filtered water. Please double-check the tank, filtered water faucet, and all fridge connections for leaks.
To make 100% sure that the system shuts off when the tank is full, verify it with the following steps:
Turn off the holding tank valve and let the system completely pressurize - which would take about 1-2 minutes. Make sure filtered water is not being used by the fridge/ice maker(if connected) and that there are no leaks.
Once the system is pressurized it will automatically shut off and you no longer should hear the water going into the drain.
If the system doesn't shut off within a few minutes, it might indicate that the home water pressure is too low or that the shut-off valve is faulty and needs replacement.
Turn the shut-off valve on the holding tank back to the "on" position to return to normal operation.
How full the tank gets varies with incoming line pressure to the undersink reverse osmosis system. If there are no leaks, it might seem like the system is randomly turning on. But, in reality, the well-pump might have turned on or water line pressure has changed from the municipality.