good question kate. Think about a computer/laptop/cell phone, etc... When they are in standby mode, they are still on and using energy. the difference is, they are using about 1/100th of what would be needed to fully power the equipment. That is why you have 6 hour talk time cell phone batteries and 300 hours of standby time.
If I had to guess, I would say the battery is still discharging while not being used because something must power the sensor to know when to turn on and off. This doesn't mean its discharging by leaps and bounds, just a little.