Possible Cause #1:
Water vapour from the air may be condensing as the room temperature drops.
The unit may not actually be leaking. Water vapour from the air may be condensing at the base of the unit as the room temperature drops
NOTE: Humidity is a measure of water absorbed into the air. Warm air holds more water that cold air. If humid warm air cools, water may condense from the air onto the floor or walls and windows. Think of dew in in your yard or your bathroom mirror after a shower.
Possible Cause #2:
Base not emptied during refill.
When filling the reservoir always empty the base before reinstalling the reservoir. Always fill reservoir completely when filling.
NOTE: The water feed system relies on gravity and vacuum in the reservoir to meter water flow to the unit. When ever the reservoir is installed on the base a certain amount of water must come out of the reservoir to create the necessary vacuum. If the base is already full then that water may end up on the floor. If the unit is only filled when the empty light comes on, this will not be an issue as the base will already be empty.