Air suspension sometimes deflates

[X5Kiwi]

I have tried searching the forums for this, but no luck so far.

I have been having intermittent problems with my air suspension (I only have rear air suspension), sometimes when I come back to the car I find the rear suspension is almost entirely flat, especially on the right rear.

Sometimes it is flat less than an hour after driving it, yet other times I can leave it sitting in the garage for a whole weekend and it stays pumped up perfectly.

If it was a leak, surely it would always leak? Perhaps there is some control valve that might not be closing correctly when I turn the car off?

I have not had any warnings on the dash regarding the air suspension being inactive. As soon as I unlock the car, it pumps back up to normal height with 30 seconds or so.