Evaporation line?

Have been taking tests all day, this line shows up in each AFTER the 5 minutes. Is it just an evaporation line? It would make sense since my LMP started Jan 31st. Please let me know your thoughts!

Have been taking tests all day, this line shows up in each AFTER the 5 minutes. Is it just an evaporation line? It would make sense since my LMP started Jan 31st. Please let me know your thoughts!