Putting things under a seat is suspicious. So is looking at security officers... or avoiding looking at security officers.
The criteria that the TSA look for was leaked last year.
Here are some of the "suspicious" factors which count against you...
Arriving early/late
Avoiding eye contact
Initiating eye contact
Maintaining eye contact
Yawning
Fidgeting
Clock watching
Head turning
Shuffling feet
Leg shaking
Touching face
Touching upper body
Rubbing hands
Wringing hands
Trembling
Exaggerated grooming
Repetitive grooming
Using hands to communicate
Insufficient body movement
Passes object to someone else
Accepts object from someone else
Pale face
Flushed face
Excessive blink rate
Lack of sufficient blinking
Excessive breathing rate
Visible swallowing
Arterial movement
Rigid posture
Arms close to sides
Body odor
Sweaty hands
Bag heavier than expected
Bag has unsuitable appearance
Baggy clothing
Looking at screeners
Looking at other passengers
Looking around the room
Looking at scanning equipment
Looking at security cameras
Looking at luggage
Looking down at floor
Crying
Laughing
Smiling
Whistling
Excessive talking
Hesitation or Indecision
Excessive swallowing
Dry mouth or lack of swallowing
Change in voice pitch, rate, or volume
Talks loudly
Talks softly or covers mouth
Delayed response to questions/orders
Focused or Distracted behavior
Ignores questions
Focuses on or repeats questions
Gives answers which are too vague
Gives answers which are too detailed
Complains about screening process
Perspires before or while being scanned
Wearing unusual or inappropriate clothing
Wearing clothing similar to other travelers
Having same luggage as another traveler
Gripping baggage tightly
Placing hand inside luggage
Demonstrates situational awareness
Interest in security personnel
Interest in security process
Lack of interest or contempt/arrogance
Appears confused
Appears disoriented
Appears disguised
Asks questions about security process
Does not respond to commands
Possesses almanac
Possesses blueprint
Possesses GPS unit
Possesses flight manual
Possesses scuba manual
Possesses liquid or gel
Possesses prepaid calling card
Possesses rope
Possesses wire
Possesses duct tape
Possesses batteries
Possesses electronics
Is single
Is male
Is under 65 years old
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No, I'm not making this up.