Faded?

Has a real positive ever faded for you? We saw the FAINTEST line but it was there! But my phone was dead and so I had to wait to turn it on and take a picture, however when I could take a pic the line was gone. The line showed up at the time the instructions said to look, so the test was good and done and nothing seemed wrong. There was color to it as well. But now I can’t even take a photo because it is basically completely gone. What does this mean?