@Jmann
She stayed behind because she wanted to be around him more. Also because she got a little jealous that he was having a secret convo. apart from her.
Then he suddenly vanishes and she walks home alone feeling like an idiot for staying late and potentially getting in trouble with her parents.
Of course Komura waaaay overplayed what he thought would happen to Mie when she went home, which led him to hide. Then regret hiding. Hopefully this never happens again and it's a lesson learned.
@Grief242
Because he couldn't convince Mie to go home, then saw the worried texts from her parents, so Komura decided to hide hoping that Mie would go home if he wasn't there.