I keep running into this flag (the same as the NPC ones). It is red with a blue star on it and I am positive it is not just some banner. I looked it up on google and the terraria wiki but I have no idea what it is. Anyone know?
It appears that people have found that this flag is linked to where the Sleeping Angler is.
From this forum:
It's a symbol that shows when the angler is sleeping and if you look you'll see it's right above the angler! But when you wake the angler up it disappears!
I've also found a video that shows this banner above the Angler and how it disappears once you wake him.
It will either be attached to a floating island if one is above the Angler, or it will be hanging in midair from the edge of the top of the map.