One thought…. if it was done after you took delivery, you would have noticed the damage, as it would have been noticeable i.e.: taken the layer of dirt & the paint off at the same time. Whereas, because you hadn't spotted it until you washed it for the first time, could indicate it was there before you picked it up?
Really, the dealers should have had the car inside & dry, so you could inspect the paintwork. Not saying it's suspicious, but just makes you wary.