[he looks thoughtful about it....]

Well - okay. So let's talk about other options. If Omi-san's foaming at the mouth, that implies poison. That means he had to have eaten food that someone slipped poison into, even that Omi-san's normally the one who cooks, right?

So... Is there anyone who's cooking he would've eaten? The stove was hot and the dishes were cleaned - so that means a meal was prepared.