In my game, just inside the entrance to Vah Medoh, slightly above head level and before the column of rising air, there's a greenish column of light (video, image).
It appears fixed in space relative to the dungeon - I can look at it from different angles, etc. It seems like maybe it's only there at night, leading to some speculation that it might be a weird glitchy copy of the light from Mount Satori, but I'd really like to know for sure before I beat the boss, given that afterward I won't be able to get back in.