hide, keep secret, hideaway
閟 centers on concealment: hiding something, keeping it secret, or withdrawing to a hidden place.
閟 combines 門 (gate) with 必 (certainly), likely suggesting something kept securely behind a gate. The exact historical development is uncertain.
A gate 門 with 必 inside: imagine a gate that must (必) be closed to hide something secret behind it.
For ヒ, picture a hidden key (key -> ヒ) kept secretly behind the gate 門.