Backgrounding
Discussing the fact that one of the plotters at JFK airport was an employee of the airport, terrorism expert Neil Livingston said "It's really hard to background these guys."
No it's not.
If you see an employee down on the GROUND with his BACK in the air, facing east, and he seems to do this several times each day, you have all the background you need.
Lame puns aside, if his name is Abdul and he insists on taking time out to pray 5 times a day and he forces the airport to build a special foot-washing room and he insists that the airport cafeteria must serve
halal food, and....
Well, you get the idea. It's not hard to background these guys at all, if you recognize who the enemy is.