IO540,
You are quite right. I know that some of the most dangerous, and I use the word advisedly, infringements have been with aircraft that do not have a transponder of any type and the controller has been correct to assume that the aircraft was outside CAS. A microlight at FL80 for istance passing 0.16nm from a 737 and controllers only knew it was there when the 737 pilot called the traffic passing his 2 o'clock. I don't believe he would have been able to avoid it if it had been 12 o'clock...
