Jamieson, Fausset and Brown on Isaiah 29:15

15. seek deep to hide--rather, "That seek to hide deeply," &c. (compare Is 30:1, 2). The reference is to the secret plan which many of the Jewish nobles had of seeking Egyptian aid against Assyria, contrary to the advice of Isaiah. At the same time the hypocrite in general is described, who, under a plausible exterior, tries to hide his real character, not only from men, but even from God.