Similar thing happened with me when I took my car for a shower/service at a petrol pump... they took a long long time due to queue and till then it was a bit dark so I couldnt properly see but next day there were quite a few areas on the bonnet where paint was wiped off in the form of dots, as if some liquid dropped over the car. Initially I thought these are just something that can be scratched and the paint will be beneath it, however to my horror the paint was removed from those areas.
Took the car straight to that petrol pump, the people there totally refused any part and said that it looks like some acid drops have dropped on the car and we don't deal with acid. They instead said that my car may have been parked outside and something dropped over it - maybe AC water. Told them there is no way anything else could have been on that car as it was parked in my garage! However, they were all thugs there. They knew that there is visible damage to my car due to their 'service'. They keep on saying that this is the first time someone has come up with this complaint - daily they deal with hundred of cars, we gave the car to you at that time you have seen it was in perfect condition, something must have dropped on the car after you went etc etc.
To top it all, some of the customers who were there at that time listening to our argument came to the rescue of car wash guys with their pathetic irrelevant unneeded comments that they have been giving their car for service there and there has never been any problem. Had to go back as the arguments were going nowhere and those people at the pump didn't have any shame to admit.