Heh, the real "Catch 22" of this event is, if you had not done the service, but instead had brought the car in to them for the service, do you think the guy who was changing your oil and discovered there was no filter, would have contacted you and said, "Hey Man, guess what? There was no f-ing filter in your housing! Can you believe THAT shit? Hahaha, we are such doofuses!!"
Nah, he would have just stuck a filter in there, and said nothing. He might have shown it to the Service Manager, who might then check the records to see if they did the previous oil service. When he saw it was his dealership that did the service, he would have told his employee to just complete the service, and say nothing.
1000% this.
It was the first thing I thought of. How do you prove it.
At least on a non-German car, lack of an oil filter shows itself IMMEDIATELY.