Well since your cars are virtually new, with the exception of your 124 which still isn't THAT old and it's low mileage, you might not see that on your cars. Especially on the newer ones with all the plastic shrouds and covers. On my car, and I have seen it on others, some sort of scaling or flaky stuff forms on parts of the motor, most notably unpainted or silver parts. It's really apparent on the accessories and some brackets.
'86 420SE Euro
904 Midnight Blue, Gray Velour
Dad bought it new, now I own it.
"A Mercedes-Benz is like a fine wine, it only gets better with age."