(W220) 1999 (UK)
The misting up happens wherther the car is at a stand still i.e parked up with the enging running or while on the move.
There has been no signs of any refrigerant leaks, loss of antifreeze but the smell is like mildew/musty adour.
The vehicle is parked up for long preiods of time due to me working overseas so the point made about water in the HVAC housing could be a valid point!
Out of curiosity how does one identify if/when the system needs regassing!
I'm not familiar with a W220 but there are several ways to figure out if the AC system is low with refrigerant. At some point it would not start (likely indicated by the light on the AC button, assuming the car has one, my 221 does not while the other cars do). You would likely have a test mode where you can read the AC pressure values.
But if the system appears to blow cold air at summer time, it should be fine.
I would consider mildew more of a problem when the car is driven often at warn, humid conditions. More water often collected to the heater core bottom. But a 1999 car could have that issue from normal use. I'm not sure if the heater core in this car leaves more often water to the AC housing than others (there was an issue with some 221 housings that were later on improved).
You could always check the heater core drain pipes (easiest to do at summer on a hot and humid day by checking that water comes out through the drain pipes).
If it sounds, or preferably smells like a mildew issue, you could have the AC system treated and see if that helps (for some time at least).