It was one I read a few years back and has been on my mind several times over the years but I can never find it.

I'm pretty sure it was shoujo and I feel like it was probably a one shot not sure if it was okay or an anthology or an extra in another story...

I believe it was about a boy who meets a girl who's searching for her senpai by finding the locations of pictures that her senpai took that I think she has in her phone cause he sent them to her or something? And for some reason or other the boy starts helping the girl find the locations of the pictures but as the locations get more dangerous he starts questioning the whole situation cause she says her senpai cares about her but if that were the case why would he endanger her or something like that and at some point towards the end he finds out the senpai she's searching for died and she knew that but was unable to move on or something this is about all I remember I'd really love to find it and read it again or it might just haunt me forever...

is it your letter?

No rip pretty sure it wasn't a manhwa and I think I read it probably a few years before Your Letter was released.

Is it air koi from https://www.mangaupdates.com/series.html?id=53784

YES THAT'S IT THANK YOU SO MUCH