When it comes to critical shyts like this in games, I wish the games did a better job (or rather a job at all in this case) of at least hinting to you about such a criteria needed to get the weapon. I don't mean it has to outright spell it out for you, but ionno...something like, "I lost something here some time ago and maybe someone picked it up" etc. I hate having to resort to strategy guides to get info on such things that in now way, shape or form are pointed out in game.
I rather not use strate guides and/or walkthroughs for my playthroughts unless I'm frustratingly stuck on a part/boss/section for hours. I'm cool with looking around for hidden stuff and there being secrets to be had, but there should be some clues on a secret at least even existing somewhere, ya know.