“It would take a lot of time to move these crates, and I doubt scattering the stones on the lakebed would be quick. If they had so much time to prepare, why hide the message like this? Why not simply put a letter in the storeroom?”
It does seem inefficient, you agree. So perhaps the arrangement had some other purpose. Perhaps they were setting it up, and they didn’t expect to have time. So they wanted their contact to finish the pattern. In which case, the natural thing to do would be for the two of them to finish it and see what happened.
And that is what they decided to do, the Loremaster continues. It took them some time. And then, nothing happened. Not as far as they could tell, anyway. With no further ideas, they decided to leave. And as they closed the trapdoor, and reached the surface of the lake, it soon became clear that something had happened after all. Dozens of coalcrabs were moving the blockage, and a beetle as tall as a grebling stood before them when they reached the shore.
The message was actually for Qlat, you realize. Once they moved everything into place, he noticed and sent his servants.
An interesting guess, the Loremaster replies. We shall see how events play out. In any case, the beetle motioned to them, then started scuttling away. With no other obvious path before them, they decided to follow it.