Welcome to Just Answer and thanks for your question. If this was a lab test via GC/MS (gas chromatography/mass spectrophotometry) there is nothing that will cause a false positive. I am assuming this is the kind of test it was, since screening tests are only positive for opiates without specifying which opiate. It would take GC/MS testing to differentiate between Fentanyl and oxycodone, for instance.
In a dipstick-type screening test there are a number of things that might cause a false positive like lots of poppyseed bagels, most prescription pain medications, cough suppressants with dextromethorphan, etc. However, a screening positive is supposed to be sent to the lab for confirmation testing via GC/MS to determine the exact nature of what is causing the positive. There are no false positives or false negatives with the GC/MS tests.
No, Fentanyl won't be in your system 7 years later, or even 7 weeks later. Perhaps someone has played a sick trick on you.