Recently I saw an unfamilar mark in a PCB made by someone else. In the screenshot, the grey is the Grid Origin, the Red is the Drill / Placement Origin and the white mark on the right is the mark I am not familiar with. It disappears when I hide the Edge.Cuts layer (which is weird), and when I examine it’s properties, it only shows: Target Properties, with some options to set. This thing also grows and shrinks with zooming, so it has a “real size” (which you can enter in the box), and this is unlike the other marks, which stay the same size while zooming in or out. I think OP is referring to this thing, whatever it is.
It’s also not a feducial. Feducials are “normal” footprints, and you get footprint properties when you attempt to edit them.