I suspect someone at work of taking cocaine but if I swabbed the toilet next to his office and it came up positive could it just be from something like a banknote?

Would a positive test indicate that drugs had definitely been taken in there or perhaps by this person at a party a week earlier and it was still on their hands?

If it was positive how long would it be in their system for a real drugs test to show?

I am trying to say that are all say office bathrooms full of traces of cocaine from banknotes etc or would it be a "smoking gun" of proof.