The possibilities are horrific. Passenger 3A is on a medication that dries out the membranes, resulting in dry lips and eyes. He's also nervous about flying. Take-off scares him, ascent scares him, and turbulence scares him, plus he has an itchy ear from the mosquitos that he left behind in Houston. They gonna pick him up?
How about putting up a newspaper to block the camera while the evil passenger is delivering signals or being nervous?
Lets add this up...your computer on board identifies 8 middle-eastern guys on the plane...who meet the eye-blinker alert. This shows up in the drive up to the runway area. You as the pilot cite engine issues and pull back to the gate. They unload the crowd and bring the 8 middle-eastern guys over to a FBI rep who asks alot of questions and asks for their travel plans. In five minutes...their stories fall apart.