I have used some of those lead check swabs from 3M. I used them on some ceramic tile and received strange results. I called the tile manufacturer who was insistent that lead had never been used in their operations. After talking with 3M they asked me to send a picture, which I did. After looking at the picture the 3M lab told me that they did not believe the test to be positive for lead but didn't explain the color I observed.
I realized I had cleaned the floors with a cleaner which contained silver citrate as a preservative. Would the silver ions have reacted with the rhodizonic acid? What color would this have produced? I observed a pinkish color (definitely not red. More pink/purple) in some areas I tested.
I can include a picture if it would help.