I missed a log.
Essentially, the PCs continued exploring the cavern and encountered a strange man who seemed to be expecting them. He told them that what they were looking for was in the chambers, below. But that there would be others coming to the chambers. And if they were willing, they should stay in the lower caverns for at least 24 hours and defend them. (Oh, and not touch any of the treasure that they found.) If they did this, he would provide them with a special picnic basket. Additionally, he was certain that the creatures that were coming would be well-equipped and that they would be allowed to keep any of the spoils that they earned defending the caverns.
The party found the treasure and set up watches. And so they lie in wait..