Parasyte: the Maxim OST episode 13
I believe that the tune you are looking for is Next to You-Anime Version. This one is different from the Original One in the aspects of being slower and softer, and with a totally different instrument playing it and from the other Anime Version in the same terms.

Unfortunately, I haven't been able to find it in any of the OSTs. It might be an unreleased track for an unreleased OST like Krazer said, but that's anyone's guess.