There are some genetically modified events that may not be registered, or registrations may not be kept up because of the cost of that kind of process in some countries. When you measure such small quantities, an old event that is past registration or is not recognized in a marketplace could appear in a test of one-billionth of a cargo, and that can cause trade disruption if that country is looking hard enough for that single event. As I say, it could be a small contamination in the corner of a ship that some seeds are lodged in. There are a number of different ways contamination can occur at very small levels. Having a low-level presence allows for the equipment that can test at such small amounts.
On November 22nd, 2011. See this statement in context.