I tested some dust for lead using a 3m lead check swab. The test was negative, and I left the testing fluid on the surface for several days because I hadn't gotten a chance to clean it up. I know it was orange minutes after testing, and also orange a couple days after testing. It's maybe a week after testing now and I just found that it has turned pink. Does this mean lead is present?
3M™ LeadCheck™ Swabs have no shelf life. Unless the ampoules/vials are cracked, they will have an indefinite shelf life.
In other words, once the vial is broken (which you did to use the tester), it does indeed have a shelf life