If you have the full adjustable suspension your car will sag a bit if left out over night. The night time cold air will cause the air bladders to sag.

The hissing you hear when you open the door and turn on the car is the diagnostic system running it's own internal self checks on the air suspension when the door is opened. This is normal behavior. It should only happen for few brief seconds.

The suspension inactive warning will also appear if your suspension goes out of the tolerance (e.g. if one air bladder is vastly different than the other side)

You probably want to check if the air springs are leaking before swapping the controller and sensors.