Where did it happen? USA or elsewhere, in particular?

I assume that the hotel doesn't offer valet parking and that the damage could not have been done by the hotel's shuttle van.

Does the hotel have security cameras in the parking lot? You could ask to see them or ask the GM to look carefully to see whether the culprit can be identified on tape.

Parking lot dings are a fact of life, especially if you live in a city or park in public lots where other people might not be very careful. However, if it's really a dent caused by another vehicle, then it becomes hit and run (and hitting a parked car) that should be reported to the police as well as your insurance company. The police might investigate any security camera evidence or they might not care beyond giving you a minimal report for your insurance company.