snippet from Chicago Tribune:
But every air marshal knows that he or she is being watched--and often tested.
Ever since the attacks on Sept. 11, 2001, air marshals and airline pilots have reported instances to federal authorities in which individuals behaving suspiciously appeared to be evaluating onboard security, according to congressional testimony. It is believed that terrorists continue to fly on commercial airlines to and within the U.S. to look for weak links in layers of the still-evolving aviation security system.
The ploys have ranged from faking illnesses to disobeying orders from flight attendants to sit down to a few reported cases of individuals running toward the cockpit door in apparent efforts to flush any air marshals from their seats.
http://www.chicagotribune.com/news/nationworld/chi-0512080236dec08,1,7824010.story?coll=chi-news-hed
Oops, sorry should have read from the bottom up.