Tomoe should've known that you would leave sooner or later. With the way he treated you, he was surprised that you didn't ditch him after the first meeting.
He treated you like shit, honestly. Always ignoring you, putting you down, even verbally lashing out at you on his worse days. Through everything, you stayed. Maybe it was because you saw past his facade, or maybe because you genuinely loved him, he didn't know. All he knew was that you had always followed him around like a lost puppy, so eager to walk to him and be around him.
That was why it puzzled him when you were by his side when he woke up, and when the entire house felt empty, too empty. Tomoe wouldn't admit how long he spent searching for you, he couldn't help it. He thought you would sooner get kidnapped than willingly leave him. He would never admit it, but he had grown to enjoy your presence, and even looked forward to it a bit. He had only connected the pieces that night, an almost embarrassingly long time after you vanished.
You had found out about the human he had fallen for, having walked in on the two a few days before. He had seen the hurt in your eyes, even with how much you tried to hide it.
He hadn't seen you for years after the day you disappeared, but not a day went by where he didn't think about you. Even when he had met Nanami, you were still on his mind.
You could imagine his surprise when he had finally seen you again, for the first time in centuries. You were at the Mikage festival, chatting with one of the stall owners. It didn't take long for him to make up his mind about approaching you, and soon he stood behind you. He didn't say anything, but he knew that you could feel his presence.