It could be water leaking into the tank from the ground water. When the plumber installs the drain into the sump basket, sometimes he does not seal the pipe where it inserts into the tank. If the water level is high enough, it leaks into the tank and triggers the pump. In addition to that mentioned by dodgeman, your toilet could be leaking without you knowing, but the possibilities are low if you do not hear the toilet turning on once in a while.
Raymond Vin Zant