Akira didn't hear Monica make her way to the door, nor did he hear her when she leaned against it. He'd gotten a song stuck in his head while sitting there and had begun to half hum/half whistle it at some point.
Organizing and cleaning had never been something he'd found enjoyable, which was likely the reason he'd actually begun the humming/whistling. It wasn't something completely unknown for him to do, but it was something he did all of that often either.