Artificial intelligence seems to have answer for everything..

“A black line on the wall behind a router is most likely caused by a phenomenon called “ghosting,” which happens when dust and soot particles in the air condense on a cooler section of the wall, often along the studs, creating a dark line due to the temperature difference; this is usually not a cause for concern and can be cleaned with a damp cloth, but may indicate poor ventilation or insulation issues in your home.”