Returned Loaner car and receive a call later about a scratch

[NMWX5]

Ive only had my car for 4K miles and I have already had many issues with it. I've already brought the car in 3 times in the past, and this time is the 4th. Reasons were: oil leak, passenger mirror stuck (module not working well), transmission bang problem, along with a recall.

After a couple days, my car was done and I returned the loaner and picked up my car. Three hours later, I receive a call from them saying that they found a small scratch on the front bumper. I talked to the person that deals with the loaners and he said he got an estimate of $400 to repaint the bumper and I have to pay.

Given the fact, I had already left and they didnt point out the scratch when I returned the car, can they still make me pay for a scratch?
How can they prove that I did it or I was responsible for it since I already left and they cant even show me what they are talking about. For all I know, the worker that took the car from me, scratched the bumper afterwards. They said I can go back to see the car and the scratch but I was already back to work and 45 miles away. I havent paid yet, but Do I really need to pay?