One firm, sends all of it's samples off to a second medical contractor for screening so it could be that. Just routine.
If it's "second stage test" then that's possibly more concerning. That could be trace amounts which need investigating and explaining.
> Poppy seeds do show up as opiates, even after moderate amount. The claim you would have to eat bucketfulls is probably false - it depends on what the threshold is. Another one to watch is Co-Codamol, it also shows up as opiates (ie same family as Heroine).
> A regular fail is THC - Cannabis. This, unlike far harder drugs stays in your system for comparatively greater lengths of time - as much as I detest it, it's a cruel twist of fate that the least harmful drug is most likely to cause a failed test.