Totally Sand:
Perception: 1d20 + 4 ⇒ (12) + 4 = 16
Not quite seeing what was dropped but eager to catch his prey, Sand darts straight forward and springs up, over, and in front of the thief.
Acrobatics: 1d20 + 7 ⇒ (16) + 7 = 23
Double move, potential consequences to come.