The one item that often features in "false positive" reports is a tea leaf strainer! Plastic ones are OK, but metal globular ones, looks very similar to an item on the prohibited list (gas related) and so cause difficulties, even though they are typically only about 3 cm in diameter. A year or two back Landrover produced a new type of car key fob which threw security for a few weeks, but it's now well known. Plasticine is another awkward one on the organic side of the house.