Correct. It gird back in the loot section. The rulebook is clear that the only way you acquire loot is as a reward. And since there are no checks to acquire on it, you could not encounter it and succeed at acquiring it, even if you did mix it with other cards.
Potentially there could be a scenario that said the reward was any loot from a previous scenario or adventure, but short of something like that, once you don't keep the loot, it is gone.
Seems like a lot more of a loophole, even though it is to the letter of the law. Mike seems to appreciate the spirit of the law and what is good for the game and the people playing it. I have always like that about this game. It is the whole reason that they don't really care about house rules as long as it makes the game more enjoyable for you.
I think if you realize that you should have taken a loot and didn't, that you can just grab it if you need it. That is how i would play it. My group often values loot very highly and never get rid of it unless it is pretty useless.
I suppose if you wanted it back later, and you were concerned about maintaining the letter of the rules, you could have a new character run through the scenario that rewards the loot. Then you could play that character with your normal group and trade over.
I have no problem with that.