One of the things that they try to emphasize to us during X-Ray training is that you CANNOT allow yourself to be distracted by something you recognize in a bag. For example, if you see a swiss army knife in a bag, don't mark it, search the bag and let the passenger go. You have to finish analyzing the image first. Seems obvious but if you think about it, it would be easy to forget that you're not done after spotting someting when you've been on your feet for however many hours. This principle applies to more than just x-ray technique and it sounds like this may be a case like that I hope you reported the incident, because doing so doesn't have to be about getting anybody into trouble but rather improving security procedures/training on the whole. Stories like this are embarassing