Seriously need help... this is insane

So first of all. I am not shitting ya. This has my brain just doozled. I bought a 3 pack of pregnancy tests last cycle. Used 2. Saved the third. (Not preg). They are first response quick results that show yes or no. Anyways. I was cleaning my desk today and behind it was the last pregnant test I had. Used. Says “yes”. My kids are 10,12, and 7. The test strip isn’t yellow, doesn’t smell like anything. Kids are denying playing with it. But it also was drawn on with sharpie so I know they were. But what would make this test say yes? Btw. The kiddos are boys. The girl is 7.