Faint colorless line

Brittney
So question. I just took a First Response Early Results test and within a few seconds a faint line appeared when i held it towards the light. Now i figured its probably the light thats making the line appear so i looked at some older tests i had and i dont see the line at all in any of them. What could that mean? If i zoom on my phone i believe i see the slightest line. Thank you!