the logic is simple - when the car is shut off, in about 16 minutes most modules go to sleep, including the EHCII, the height module... while the car is asleep, it is possible for the suspension air bags to leak out air, fast, slow, doesn't matter. when the car is awaken, either by the opening already not locked door, or using remote to unlock, or by any other manner (say, opening trunk lid), the car wakes, up , the modules wake up and prepare for action - the EHCII in this case is checking the hieght levels of all 4 corners and if it finds them to be off, it is using air inside a compressor holding tank to inflate the bags to prescribed height. The pump is not activated when the engine is off, so the air is coming from a holding tank. this symptom indicates that there is an issue with air bags not holding air overnight, no issue with holding tank, as it has the air stored, and no issue with the pump. the bags can be replaced using the quality Arnott bags, that many users here on the forum found to be very good quality.