I can testify from a personal occurrence involving a member of my family at an airport in Florida less than a year ago (domestic flight btw) that this document check can also be circumnavigated if one chooses the right moment. In our case there was no deliberate intention. It just happened during a busy period when the agent was temporarily distracted, as was said member of my family who didn't realise they had sailed through unchecked. Said person did a 180 to ensure that the check was performed and to get the appropriate "mark". It could well be she simply waited until a chance presented itself.