Running.
It was hard to tell if they were running away, or running after. The night was dark, thunder rolling in the distance, but fast approaching. They really shouldn't be out here in this dark woods, somewhere in Southern United States. But there was a job, and the Winchesters weren't ones to slack, especially when there was an unidentified creature lurking just at the edge of their vision.
So the two brothers ran. Sometimes something would come up behind them, forcing them to run faster, sometimes they almost caught up to the unidentified mass, but only to have it escape observation again.
It wasn't until the rain started pouring down, and they had finally lost track of whatever the thing was, that the two brothers decided to hole up in a cave nearby. Hopelessly lost in the woods, they decided to wait until daybreak to find their way back. Inspecting the cave, the younger noticed a strange hole in the floor of the back of the cave. Calling the older over, he tossed a rock down, listening for the sound of it hitting the ground. No such sound ever came.
There was a sudden shove from behind, and suddenly, both of them were falling.