4And the men of David said to him, “Here is the day of which the LORD said to you, ‘Behold, I will give your enemy into your hand, and you shall do to him as it shall seem good to you.’” Then David arose and stealthily cut off a corner of Saul’s robe.
David’s plan was to take a piece of Saul’s robe as proof that he had it in his power to kill Saul if he wanted to.1 Presumably with Saul’s robe being thrown across a boulder on the cave floor, David was able to approach the robe and return to the inner recesses without being seen.2
4 And the men of David said to him, “Here is the day of which the LORD said to you, ‘Behold, I will give your enemy into your hand, and you shall do to him as it shall seem good to you.’” Then David arose and stealthily cut off a corner of Saul’s robe.