I almost exclusively buy one-way tickets -- sometimes the day of the trip and sometimes months in advance. I have not been tagged for secondary security in a very long time.

RKKWAN, did your sister buy her own tickets? And did she have her frequent flier number on her reservation? I ask these because my wife and I have noticed that she seems to get tagged for screening a lot more often when she flies on our companion pass and I pay the security fee. It's possible that when the only passenger-specific information in the record is the passenger name, the computers raise the red flag.