Grin makes his way along the southern cavern wall without finding anything. When he reaches the opposite side of the cavern from where they entered, he finds an opening (which they already thought was there). However, he finds a distinct lack of anything even hinting at goblins on this side. He does see that some activity has happened at the opening, but they don't match the goblin activity they saw on the other side. In fact, they look as if those who made the tracks treated the pink slime as a barrier. The opening does seem to continue on down as far as his torchlight will carry. However, rather than exploring on his own, he loops back around along the northern wall. About a third of the way back, he sees a relatively small opening that does show signs of goblin activity. Presumably, this is where they put on their boots to cross, so the marks of activity are quite evident. Humorously, the opening is impossible to see from the side of the cavern from which they entered. In fact, if Grin had moved around in the opposite direction, he could have missed it. (Well, not Grin, he's a dungeoneer!) But, since he was approaching from the "correct" direction, he could easily see it.
He quickly waves Ishzu over and they both take a look. They agree this is likely where the goblins entered from.
OOC:
So, what do you want to do? You have a few obvious options:
1) Explore the goblin passage.
2) Explore the passage on the opposite side of the cavern.
3) Go back, and check out one of the other open mysteries in your set of caverns here.
4) Go down to the deeper underdark.
5) Skin the mimic and make galoshes for everyone!
Remember that your base instructions are to finish clearing the caverns around the Maxtrix Node and identifying entrances. You have done that here now. But these are general directions, not orders. You can go wherever you want.