—it doesn't matter. It doesn't negate the fact that you came up positive the first time. You still go through an interview to discuss where it is and why you might have picked up some drugs.
The outcomes on this are vastly different depending who you are. In 2011, the Honourable Kim Pate—many of you probably know her personally—testified to I believe the Senate committee when they were looking at drug interdiction that she had rung positive on the ion scanner, and they had spent a great deal of time going through what she might have touched or what she might have taken. They decided it was the Dimetapp she had given to her kid the day before that had caused her to ring positive. She was allowed to go in.
If you're a family member, they don't go through all of this rigmarole and try to give you the benefit of the doubt and figure out what exactly it is that might have set the thing off. They just say, “You have set it off and this is a problem.” They will look to see whether you have set it off before. If you have, they will say that this is evidence of a recurring pattern, and you're even more likely to be denied your visit, perhaps even to have your security clearance revoked altogether.