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.
Just seen this.
Not saying they would, but speaking from experience (hate to say I've done it more than once), if I had a new/used car going out which had damage, I would make sure it's out in the rain, or parked tightly to a car so you can't check that side, or park it in the shade. I would usually walk round the car with them and conveniently place myself in front of the damage so they couldn't see.
Not saying this is the case, but you never know. If it was done at the dealership, someone will have known about it. Cleaners are very good at telling sales when a car is damaged, then it's in sales hands.