Publicintelligence.net has posted several FBI bulletins to employees in various fields (hobby shops, rental trucking companies, hardware stores) about how to spot suspicious/terrorist-y behavior. But my favorite one is to the tattoo artists of America.
Under the "What Should I Consider Suspicious?" header, the FBI advises:
At the bottom of the bulletin, the FBI reminds us:
Call me crazy, but that disclaimer seems a little insincere, given the rest of the verbiage.