In the case at hand, the player did not claim his pass was unintended, he said it "came out separately". Separately from what? Not the 3NT card, surely. On the evidence presented (which I grant is one sided and self-serving, but we don't have the player available to answer questions) this was not an unintended call, but it's not because it lay on the table for a time.
One possible scenario here is that the player was thinking whether to Stayman, or just bid 3NT, had a momentary lapse, and passed. At that moment, he thought, however briefly, that pass was the right call. So he can't change it under 25A, and can change it under 25B only if his LHO accepts the change which I gather had about the same chance as the proverbial snowball in Hell.
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