As for the PD, if you think they don't know the majority of people who are carrying concealed, you are only fooling yourself. Many people whom thing they are concealing their handgun in fact, give off signals to anyone who is aware that they are carrying. A touch or adjustment of their weapon, even if with the elbow is a tell.
A list of things I look for when playing the "who's carrying concealed" game.
-Pulling down a shirt or jacket.
-Odd or inappropriate clothing for conditions. Vests/jackets in hot weather for example.
-Fanny packs when pants have plenty of pockets.
-Fanny packs that obviously have something heavy in them.
-One arm kept close to where a holster probably would be. This arm doesn't swing freely with walking motions. Shoulder holsters generally have this "tell" more often than waist holsters because shoulder holsters tend to flop around a bit.
-Hitching up trousers often. There is a reason those pants keep falling down. Could be suspenders are needed or it could be something heavy on the belt or in the pocket keeps dragging them down.
-Exceptionally long pant legs with one leg where the material doesn't "flow" as well as the other when walking. Indication of a possible ankle holster restricting the pant leg's motion.
-Elbow checking gun.
-Belt at an angle. Possible weight pulling it down.
-Sagging pants or jacket pockets.
-Reluctance to remove a jacket when inside a building.
-One arm furtively holding a shirt from riding up when reaching for something.
-Shoulder going up before exiting a car as the gun is either reholstered, checked for positioning, or clothing adjusted.
-Keeping one side of the body at a slight angle away from the person in front of them.
-A hand goes into a bulky pocket, and stays there.
-Hugging with arms under the other person's arms forcing their arms away from bumping a gun.
-A dull "thud" if something is bumped into.
Let me emphasize that these "tells" are not proof positive that someone is carrying a gun. But they are hints that many concealed carriers project to anyone interested enough to look for them.