Nightmare situation.
My vehicle was broken into in a supermarket carpark.. no cameras, and the supermarket take no responsibility... in fact they seem to have a policy of not having cameras on the car park spaces.
The fact the sign was missing might not matter in the end. You didn't pay for parking,.. so there was no contract, or agreed service level etc, .. it's not really reasonable for you to form the view that your car would be safe. It's tricky, .. I'm not sure about a hotel carpark, or even about a normal, paid, underground carpark.