Please could you tell me what makes a person suspicious?
the way he looks?
is he nervous?
does not look forward your face?
Thanks for your help...
There is no ONE answer to this question.
And, there is more to the calculus, than the behavior of another person.
On one side of the equation, is the police officer...his or her experience, training, maturity and general grasp of the totality of the job.
You might want to review an old USA Supreme Court Decision on this subject. I believe it is Terry v. Ohio 392 U.S. 1 (1968)
This case spells out experience levels of the officer, as it pertains to his advancing formal contact with citizens who acted suspiciously.
As for the citizen or person, their behavior may be furtive, may be hanging around a closed business, may have bulges around the waist-line, may seem nervous, be in a business district at non business hours....or be in an area not usually associated with pedestrian traffic, e.g., loading docks, staggering, bloody, disoriented, a person's demeanor and clothing not fitting the car they drive, a person who does not fit the neighborhood, and so on. You get the idea...something that draws the attention of the officer.
Old time cops, refer to this as a "JDLR" (just don't look right)...
A seasoned officer in a marked police car, like a "black and white" cruiser...driving down a street where typically it is known where drugs and prostitution can be commercially active....EVERYBODY on that street will have their eyeballs glued to that police car.....except.............a person who is wanted, with outstanding arrest warrants pending, or somebody holding contraband...drugs, pistol, X-felon with a pistol something, where the suspicious person KNOWS they are 'dirty'....however, typically, they will try not to look conspicuous, by pretending not to notice the police cruiser...which makes them suspicious, as they are the only set of eyes on that street, not fixed on that police car.
Anything out of the 'norm", something that does not fit, manner and attitude.....as you can see, the process for the officer to make these determinations, is not as abstract as you might think....
More depends on the officer' skill sets, than the JDLR person...
Hope this is helpful.