Sitting behind rows of computer screens in a command center in Riverside, California, specially trained Customs agents keep a constant watchful eye over most every GA airplane flying anywhere in the country. What are they looking for? "Suspicious behavior," is the official answer. What does that mean? According to CBP, a suspicisus airplane is one that flies from one uncontrolled airport to another, isn't on a flight plan, flies low, or flies long distances before landing – in other words, all the perfectly normal and totally legal types of operations general aviation pilots engage in every day.