The International Association of Chiefs of Police training guide on profiling suicide bombers gives some guidlines as to how a terrorist behaves:
I exhibit all of these traits!
?¢‚Ç¨ÀúBehavior. Does the individual act oddly, appear fearful, or use mannerisms that do not fit in? Examples include repeatedly circling an area on foot or in a car, pacing back and forth in front of a venue, glancing left and right while walking slowly, fidgeting with something under his or her clothes, exhibiting an unwillingness to make eye contact, mumbling (prayer), or repeatedly checking a watch or cell phone. [...]
Appearance. Is the clothing, grooming, gender, or age of an individual out of place within the context of the environment? Examples include someone wearing a heavy coat or jacket in warm weather [...]
Equipment. Does a briefcase, duffle bag, or backpack seem extra heavy or have protrusions or visible wires? When the individual sits down, is he or she overly protective of this item or preoccupied with it? [...]?¢‚Ç¨‚Ñ¢
In London a french techie was arrested in a tube station while waiting for a train because he was “suspicious”. Even through he was innocent his fingerprints, DNA and other records will be kept by the British Police.