When Batman is fighting the parademon in the beginning of Justice League, he catches the demon with a net on the wall. The demon explodes after some screaming and twitching, leaving goo picturing the three mother boxes symbol refenced afterwards in the movie.
Why did this happen? We could say it is just trying to increase the mystery about the boxes in the beginning, because in hindsight there is no reason why an exploding henchman would leave these as a mark.
The other depictions I can remember do make sense as they were historic "aritfacts" of some kind.
Am I missing something or is this just bad writing?