Efflorescence on my paint job after garage storage?

My W140 has essentially a perfect original paint job,silver color, and the car is stored in my garage most of the time.
When I went in to check it this week, I saw what I thought was moisture condensation on random areas of each side of the car, not the top, hood or trunk. However it was not moisture, but rather a silvery material in streaks, with some adherence to the paint. I could gently wipe it off and the underlying paint surface was normal. I checked the roof for leaks above these areas, as we have had a lot of snow here in the NW recently, and then a rapid thaw and melt.The roof was fine. However I note that the cement floor has white deposits, and these are termed "Efflorescence" literally meaning a "flowering" or emergence of alkali salts from the concrete, which come to the surface with moisture, the moisture evaporates, and the salts are left behind.

The garage is beside a road that was heavy with snow, now all melted, so that could be the source of the moisture.
I have never seen this before and the car has been stored for years in this garage with no such deposits.

Any ideas of this or other causes? I am going to gently wash the car to remove them, but again I see no evidence of paint damage.