2003 Chysler Grand Voyager Motor in device under bodywork

I have been having terrible problems with the battery losing charge over 36 hours. When you switch everything off and leave a multi-meter in between the disconnected earth lead, there is next to no draw. During the night, something fires up and the battery goes flat.

Recently we found that when we reconnected the battery following an external charge, there was an electric motor, whiring noise coming from beneath the vehicle under the rear sliding door. There is a small device there with what looks like a 1/4-1/2" exhaust pipe-vent. Since disconnecting this the battery has kept it's charge.

What is this thing and why would it come on in the middle of the night when the car is locked and everything turned off?