Is there anything that could cause a false positive drug test for fentanyl?

No. Conventional urine drug screens (uds) look for metabolites from naturally occurring opioids (like morphine and codeine), but fentanyl is a synthetic opioid which is metabolized to different products. These products are not picked up by uds.
No. Fentsnyl has no cross-reactivity with other opioids tested in the urinary drug screen.