Surveillance - The unusual recording or monitoring of activities or security measures with a camera, note taking, or marking of maps etc. 

Elicitation - People or organizations trying to gain information about infrastructure, people or military operations without a valid purpose or authority. 

Tests of Security - Improper attempts to measure reaction times or to penetrate physical security barriers in order to assess strengths and weaknesses.

Acquisition of Supplies - Purchase or theft of explosives, weapons, or ammunition; acquisition of military or emergency responder uniforms or decals, flight manuals, passes or badges by unauthorized individuals. 

Suspicious Persons Out of Place - People who loiter in, or look out of place in the neighborhood, workplace, or in a particular environment.

Dry Runs/Trial Runs -  Putting people into position and moving them around according to their plan without actually committing the terrorist act.

Deployment of Assets - People and supplies getting into position to commit the act.  This is a person's last chance to alert authorities before the terrorist act occurs.

If you see suspicious behavior, do not confront the individuals involved.

Take note of the details:

S - Size (Jot down the number of people, gender, ages, and physical descriptions)

A - Activity (Describe exactly what they are doing)

L - Location (Provide exact location)

U - Uniform (Describe what they are wearing, including shoes)

T - Time (Provide date, time, and duration of activity)

E - Equipment (Describe vehicle, make, color etc., license plate, camera, guns, etc)

Suspicious activity is often recalled after an event. We must train ourselves to be on the lookout for things that are out of the ordinary and arouse suspicions.