Well, when I think of secondary, it can be more a matter of Customs and Immigration, especially if you are not a US citizen and also depending of your travel pattern (other previous trips) they might want to know more about it. Immigration might ask you questions about those trips and purpose, and they might ask details of those trips (cities you visited and what you did there, tourist attractions you visited and such). Even if you have no stamps and your passport is clean (no stamps) they still know which countries you have visited prior arriving to the US. If you are sent to a secondary screening, they might open your bags. But secondaries not only happen in the US (try Customs/Immigration in Canada any day). In my own experience and as a not US citizen I was never sent to a secondary screening in the US, so far so good for a visitor (me) who spends her hard earned money in the US. I'm just the average tourist and spend money in hotels, tourist attractions, shopping, so nothing extraordinaire to report here. I have never visited countries on "the list" though.