The PCs have been traveling for a long time down the same stretch or round with no sign of anything suspicious. As they cross over to the next hill they see a gigantic splater of blood with a bleached dragons scull in the center of it. It the dragon skull's mouth is a tattered backpack with something wiggling inside of it.
You pass through a woodland with a floor that seems remarkably clean of debris. Suddenly your attention is drawn to a twisted bit of wood by the trail side, a short broken twig with several stubs of branches along its length and most of the bark missing. When you reach it, the twig blurs and turns into a small brown lizard, caught in the middle of its body-twisting run. The little reptile darts away down the trail with impossible speed, no doubt magically enhanced, and is lost to sight in a moment. Now someone knows you are coming...