Suspicious activity is any observed behavior that could indicate terrorism or terrorism-related crime. The "If You See Something, Say SomethingTM campaign respects citizens' privacy, civil rights, and civil liberties by emphasizing behavior, rather than appearance, in identifying suspicious behavior. Factors such as race, ethnicity, and/or religious affiliation are not suspicious.
A vehicle may be parked in an odd location, a package or piece of luggage is left unattended, a window or door that is normally closed is now open, or another out-of-the-ordinary situation.
A person may question an individual or individuals at a level beyond mere curiosity about a building or facility's purpose, operations, security procedures and/or personnel, shift changes, etc.
Someone pays unusual attention to a facility or building beyond a casual or professional interest. That includes extended loitering without explanation (particularly in concealed locations); unusual, repeated, and/or prolonged observation of a building (e.g. with binoculars or a video camera); taking notes or measurements; counting paces; sketching floor plans, etc.
Be prepared to describe to authorities: