This one I go with just because enough time has passed that I believe Harry genuinely forgets about the mirror, and it makes sense. It’s something he never even unwraps (so it’s an abstract concept to him rather than a concrete object) and then hides in the bottom of his trunk.
actually think this is one of the few times in the books when Harry doesn’t go barreling headlong into danger without thinking. He tries to make sure Sirius is okay. He tries to go to Dumbledore. He even tries to get a message to Snape, and although that works, he had every reason to believe it hadn’t.