You can highly doubt it, but my comment is based on 15 years in the restaurant business with a very regular customer base at both restaurants. Probably 90 percent of the complaints I received were unreasonable at best, dishonest at worst. Someone eating half a huge burger or more before complaining that it was overcooked and requesting another burger. The couple who only came on cheap steak night, split a steak and requested extra lemons, extra crackers, extra butter, extra-everything-that's-free, then still complained that the steak wasn't tender enough, the bread was cold, the service was lacking and tipped $1 no matter what the cost of the meal. Lots more like that, kept coming back over and over but complained every time and tipped terribly.
But I would still kill them with kindness, comp the food that wasn't eaten, offer free desserts, apologize and thank them for their business. Not for their sake, but for the sake of all the decent customers sitting around them, those people who might've occasionally had good reason to complain but never did because they didn't want to be in the same company as lousy cheapskate scammers who walked into a restaurant knowing they were going to try to get something for free and shaft the server afterwards.
So I never treated people as if they were looking for something for free, even when it was blatantly obvious they were.
The truth is, most people with a legitimate complaint rarely speak up, as is the case with the OP. I would always look for signs that someone wasn't happy with the meal or the service. If I was clearing the table at the end of the meal and the food was only half eaten, I'd ask if something was wrong with the meal and I'd comp it. If a beverage was empty, I would apologize and give a free drink. If I was manager on duty and I noticed people looking around for their server, I would go to the table and ask what they needed. I took pride in my work as a server, bartender and manager and wanted customers to have a good experience.
But you're right about one thing--If I am unhappy then you should know it and know why. You can't fix problems if your customers are not letting you know that you have them. As I said, I would've complained in the OP's situation, and I think most people should complain in similar situations but at a point when management has a chance to make things right, if possible.
HOWEVER, OP said the food was fine, he ate some and took the rest with him. So as someone who's been on the other side of things, I personally would have paid and still tipped and then complained to the manager afterwards.