It must be many of clues hiding in the black shadow that appeared in each chapter.

[TrailBlazer]

I found a shadow when the enter the forest in the night, a shadow appeared between crows if you filmed before they cursed, and a shadow appeared in the tower when kate out of cabin in the night, did I miss any of them? Maybe its a important information for tape 2

[Stoibs]

I've been trying to find a list myself too.
There's atleast one in 2022 at a certain point if you quickly look out the window. I can't remember exactly when, I think around about when the hole appears during one of the flash-forwards

[GrauBlitZ]

I also saw some of them, I first thought it was a bug in front of the bar in now-time. But then I saw them more often, unfortunately I did not keep a list. Also multiple in Tape 2...

[Dragonfirefly]

There are three Shadow in the Wood at Night. One looks like Corey, behind u when u face the Tree, another one looks like Kat and its between the Trees and the Last one i found was behind the tree right side and looks like a middle age Man (could be Phil or the Chief/Master of the Coper Mine). Another One Appear when u play the video and then blackout happen, looks like Kat right side at the video player (u can´t film this shadow).

Bonus: in the Bloodymary Game, if u go to the end, u see a shadow in the mirrow (Kat).