This is bad news and something that I dread every day.
Just one thought - you say you washed it day before...are you sure it wasn't crap cought on your washmit? I seriously doubt that, but just a thought.
When I had paintwork on my E90 years ago, they said it was about £175 a panel, but obviously the roof is two panels, but each much smaller.
Although annoying, you've spotted it quickly, so can narrow down where you've been....possible cctv?
Hope it gets sorted (which it will do, its just horrible during the process cos of the bad feeling). I'd recommend a BMW approved bodyshop, then you know there won't be any warranty issues later down the road.