No expertise at all, but just to clarify for others: This happens after you wash the car yourself, right?
One suggestion: I know with the Meguiar's wash stuff I use, it's recommended that one not spray at all, but simply run an ordinary non-nozzle stream from the hose to rinse, as the gentle flow works with the chemistry of the wash solution to enhance a sheeting action and minimize water spots. Try losing the nozzle for a wash and see if not spraying water at all, but simply running it over the car as needed, makes any difference.
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)