Here is a list (and I am betting there are more) of things that can cause a false positive test on amphetamines.
What are other false/positives for methamphetamine?
I am sure there are more, and as Eskie said there is I guess the REMOTEST possibility of contamination although I can't see how on earth it could happen.
If you've ruled out all the "known" suspects and are left with your eliquids I guess you could pay to get them tested for amphetamine like drugs by a lab. It may be expensive to do so, I think it is SO unlikely, but who knows. If in fact you find one of your manufacturer's juices is in fact contaminated, well, you'd have a legitimate case for the cost of the testing being paid for, time off work, other whatever relevant harm but I really have to think it's unlikely to almost impossible but I guess you should explore all options if a known offender is NOT the case for you.
If a urine test, it would have to be fairly (somewhat) recent, and if a hair test, a hair test can show possible use for several years, depending on the length of your hair.
Good luck!
Anna