There are a couple of possibilities - you saw the faint lines AFTER the time mentioned by the test manufacturer had expired. They are evaporation lines and may have disappeared later fully.
The other possibility is that you thought u saw lines. It has happened with me as well that I want to see a line and I think I see a faint one, but it wasn't there at all.
a blood test would tell u for sure whether you are indeed pregnant or not. also, wait for a week after your missed period for proper results