Mold or rust stain on roof truss

Paid a visit to my attic earlier today to see if there was any winter damage. Looking all good except for a small piece of truss that has a black stain on it (see picture). I would have said mold but the symmetry of the stain is odd. Could this be a rust stain or something else that was already present when roof was built? If you look at roof sheathing nearby and cardboard piece preventing blow-in insulation from reaching soffit, it does not look like there was water infiltration anywhere nearby.

Looking for any input that you guys may provide.