media that is found, but its origins are unknown

Katsudō Shashin comes to mind. A fragment of it exists, but we still don't know who the artist behind it is.

Basically reverse lost media.

Gotta get the obvious one out of the way; The Most Mysterious Song on the Internet, aka Like The Wind/Check It In, Check It Out.

Now for a less obvious examples. I remember hearing that origin of the old Rayman Sex Bomb animation (not NSFW, I swear) was unknown, but if I'm not mistaken it's been confirmed to have been a test animation for Rayman 4 that found it's way online.

There are several nes rom hacks and bootlegs without any name or company attached with mysterious origins

If I'm not mistaken, the origin of that one bootleg nes game that become a meme some years ago, 7 Grand Dad, is still unknown.

my theory is that it was a recording of a discord vc where the guy said that, and then someone uploaded it.

lost media found in what