Why did Sha'ul need to search? Did they not tell him David's precise location?
Malbim: Sha'ul went to Chorshah, and did not find him there.
What is the meaning of "va'Yered ha'Sela"?
Radak: A valley was between the plain, where David was, and the rock. He descended to the valley in order to ascend the rock. This is like I explained about Emek Achur in Yehoshua (7:24). 1
Metzudas David: He descended from the rock where he had been, to dwell in the Midbar, where it is easier to slip away in any direction.
Malbim: Initially he was in the plain, where it is easy to find him. He descended to the rock that was also in Midbar Ma'on; it was a big mountain. 2
Radak (Yehoshua 7:24): There was a mountain between Machaneh Yisrael and the valley. It says "va'Ya'alu" regarding ascending the mountain.
Perhaps Malbim means that he descended to the foot of the mountain. (PF)
Why does it say "va'Yeshev b'Midbar Ma'on"? Verse 24 already says so!
Radak: This teaches that David did not have time to flee Midbar Ma'on. He ascended the rock to be protected from Sha'ul.