I believe theh must be aware of it in some form or another in order for them to gain black psyche-locks. Those people underwent events so traumatizing, they unsubconciously lock it deep inside their own hearts, effectively making it so that they're keeping secrets even from themselves.

There had only ever beem two such examples of black psyche-locks appearing. One, it was because the character in question was apparently so paranoid about whatever he's doing, he's keeping secrets even he's not aware of. The second was due to an incredibly traumatizing event.

And I believe that incidemt in Apollo Justice may had been a translation mistake.

And I actually doubt those aware of the Magatama's powers would shatter their own psyche-locks just like that, especially not if it's so,ething like around 5. The amount of psyche-locks represent their resolve in keeping the secret. The higher thr amount, the bigger the resolve. (And while a big resolve was instantly shattered when Phoenix gave the one who has is an item she really had wanted, this was played for laughs as it showed just how badly she wanted it.)

Incidentally, the Magatama is not fool-proof. There's nothing stopping a person from piling a lie right after their psyche-locks had been shattered, passing it off as them no longer hiding anything. This is exactly what The Phantom did in Dual Destinies when he had inadvertantly triggered Phoenix's Magatama.

In addition, if the person really does believe a falsehood deep inside their hearts, even if said falsehood happens to be "they did nothing wrong" {when they actually did}, they won't be hiding anything and thus, won't trigger Phoenix's Magatama. This is exactly why it didn't work at first when Phoenix tried to use it on Matt Engarde - as a sociopath, complete with all that implies, Matt really did believe he didn't commit murder {even though it was him who ordered the killing in the first place}.)