That brings up a story my daughter and I experienced in the year 2000, we were returning from Houston to Dallas on Continental Air, short flight, half empty flight.
We sat near the door midplane, she sat across from me and behind me there was a man, dark hair but not ME he acted suspicious in that he was extremely nervous, at that time I thought he may have had fear of flying, we took off and he kept getting up from his seat and sitting back down, finally he got up and took a attache case from overhead and sat next to me. He opened the case and I kept looking at my daughter to do something, she got up and went to tell the flight attendant of the bizarre behavior, I looked inside the case and he had NOTHING but a pen in it.
He kept opening and closing the case. The FA walked by several times but never did anything to him. All I can say was it was an experience I will never forget and if that happened now the guy would have been arrested.
Who knows what we may have seen before 911 and never paid attention, so being alert is of extreme importance now especially.