Context: This is picture (taken with a smartphone*) of a copy of school test. The original answers were in black pencil and the professor made check marks in red.
However, the parts of the answer that touched the red check mark, became red as well.
Looking up close at the copy it self, it seems to have been printed by a laser printer, given the small but visible gaps within the red check mark.
*The artifacts were NOT generated by the phone. They were present in the printed copy itself. Thus, either the scanner or the printer is to blame.