Every time I read Masara Minase's Aishuu I get a strong craving for this theme. It doesn't have to be part of the main storyline, but I would like it to be more than a tiny mention. Something like one person deciding to leave suddenly for whatever reason and abandoning his friend/boyfriend/etc behind without a word, and the friend/boyfriend/etc misses him or whatever and looks for him. I'm not looking for one where the missing one died or married or whatever, with the lonely one waiting at home or hoping to see him at a train station or anything (unless they somehow miraculously get together again anyway... I think there was a ghost story like that...)

They don't have to end up together in the end, but it is preferred. I'm also a bit lenient on my search requirements because I know how difficult it is to track down some themes.

There's Deep Flower by Yamakami Riyu. And also Neo Arcadia - Yami ni Saku Niji, but its translation hasn't been finished.

Didn't Masara Minase write several other mangas with a similar feel? Like, Lover's Position? I wish that had been a longer story. I pitied the uke. And Toiki Yori mo Yasashii had searching in it.