Originally Posted by Mats

According to US Customs, suspicious activity included:

- Traveling with or without baggage
- Traveling in the front, middle, or aft section of the plane
- Traveling alone or with other passengers
- Checking in early, on time, or late
- Being the first to get off the plane, the last to get off the plane, or somewhere in the middle

Originally Posted by Gynob001

What constitutes a suspicious behavior?
1. Leaving package/luggage unattended.
2. Carrying any prohibited items such as firearms/knife etc.
3. Use of threatening language or gestures towards crew/officials.
4. Vandalizing property in order to atrract/dicert attention.
5. Attempting to gain access to restricted areas.
6. Taking pictures/videos of restricted areas or general public/traffic

I am not sure how speaking in foreign language, wearing specific article of clothing, use of restroom, taking pictures of family in the airport, looking outside plane window to see "sights, etc could constitute suspicious behavior.