A no-knock lock is a (generally) non-magical lock designed to not be subject to the ubiquitous knock spell. It has four locks, set in two pairs. Unlocking one of each pair causes the other of the pair to automatically lock. The process occurs as the unlocking occurs, so at no point is the overall lock unlocked. Thus hitting a no-knock lock with a knock spell results in the two locked sections to unlock, simultaneously locking the other two sections, resulting in a still-locked final lock.

A key for a no-knock lock interrupts the connection between locking mechanisms, so if the proper key is inserted, the lock can be unlocked (by turning the key or casting knock). This interruption can also be artificially created with a Disable Device check, with a DC equal to the unlock DC of the lock.

A no-knock lock costs x10 as much as a typical lock with the same unlock DC. Some no-knock locks are magically enchanted to increase their hardness and hit points, or made of adamantine, or both.

(Of course an item like this both needs to be sauce-for-the-goose, if it works against PCs, it works against NPCs, and used sparingly. There’s nothing wrong with occasionally exploring the corners of the world defined by the Pathfinder rules, but a GM shouldn’t be a dick about it.)

Fun RAW Pathfinder fact: The knock spell does not unlock a independent lock that is not part of a door, box, or chest. (Looks at the target line. That gets expanded a bit in the text of the spell as exceptions, but anything not called out as an exception is not a legal target of the spell).

It won’t undo shackles either, unless the shackles are themselves being used to hold something shut. Locked gate or portcullis? Not subject to knock.

I suspect no one runs it this way, but it DOES explain why you still might want a rogue in the party.

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