In an MMO I think I went through an experience of feeling of inefficient hypothesis. But it didn’t feel to involve secrets. It involved dangerous areas paying out more and the common wisdom being that the reward was not worth the risk. It tended to involve stuff like new faces not previously seen in the dangerous areas were piled up on to discourage them from upping the population (with possibly the important structure that “familiar” faces were attacked based on material efficency consideration and not scare considerations). But if you lived there a little while it wasn’t unreasonbale to come to the conclusion that it is mostly free money.
You could see both efficient and inefficient market hypothesis in effect at once in a kid of tug-of-war. In one sense it might have been that players preferring a anxiety and risk free play experience extorted real measurable economic effects. So in that sense there still might not be free lunches, what you have free on one axis you migth suffer on another axis.