Has anyone experienced a disappearing line?

I took both these tests yesterday. Which I really didn't think anything would show up. It was last night after 6pm and then around 10pm. Also I'm only 9 days DPO. I know you are not suppose to read tests after 10 minutes or so with these two brands and I'm not really questioning being pregnant and I'll probably continue to test before I call my OB, but I went back to take a picture of the tests with my husband phone to surprise him in the morning. Side note not sure if I should wait till his birthday to tell him which is August 11th or just find a cute way to tell him now. Not sure if I can keep it a secret that long. Anyways when I went back upstairs which was an hour or two later. The blue test line was gone? Has anyone had this happen and was the test then a false positive?