Red dye?

Karly

Can a red dye test give you a false positive? I took a test this morning with FMU, showered and then once I got out of the shower it said negative. I threw it away and then just now I looked in the trash as I took another one and there was a faint color line, but this is the ONLY one I’ve ever had give me a faint positive of this brand? All my others NEVER gave off a faint line even by the next day?