If there were misfire codes registering then it does not matter if spark plug replacement is not on the service schedule for the mileage - they have to be replaced and if they are not I believe he cannot stamp the book and then you don't have a full Mercedes service history.
If they find a fault they have to fix it. They may have saved you money down the line and given you a more comfortable ownership - which is why you have to pay them a great deal of money to pamper your vehicle.
Noises are always irritating but they can be notoriously difficult to diagnose.
Given the circumstances I think the "unscrupulous" tag is a bit strong.
The legal situation depends on how old the car is, when you bought it etc. none of which you have told us.
These things are always better resolved by cooperation. They want you to enjoy your car and come back to them.
So talk to the dealer and see if they can find the noise for you but I expect you will get a great deal further with them if you do not accuse them of being unscrupulous in the matter of the spark plugs
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