Hakkel glanced around the Cube. He saw a hallway on the other side of the ring. But Micah had entered the Cube only moments before Hakkel had smashed through the wall. With the ring and the stands in the way, Micah shouldn't have gotten that far in only a second.
So, unless he had somehow managed to hide from Hakkel's detection within the stands themselves, the only place he could have disappeared was the hole in the ground next to the ring.
And if it wasn't, and Hakkel was wrong, if Micah had managed to cross the Cube's entire inside and gone out the other side, the two Riders Hakkel had sent would be waiting for him. Micah could also be hiding someplace else inside the Cube, hoping Hakkel and the others would assume he had gone out or something.