Another thing to keep in mind is to be asking, “What doesn’t fit in this picture?”
Example 1: Evening, full dark, I needed gas and pulled into a gas & grub. The ill-lit pumps were off to the side of the brightly lit, full glass front of the convenience store. Parked on the side of the building, next to the pumps, was a ratty old car with two young males sitting in it. It struck me – why park & sit there? If they had (legitimate) business in the store they would have parked right in front in the most convenient spot and they’d be in the store, not sitting in their car. No one in the store could tell they were there but they sure had a great spot for watching the gas pumps. I never stopped, just circled the pumps and got gas a few miles away. For all I know they were completely innocent of any bad intentions, but they didn’t fit the picture.
Example 2: Briefly, a friend of mine in a big city noticed a pair of young men who didn’t seem to fit the area. When they followed and accosted him he was ready, drew, and they suddenly had urgent business elsewhere. His initial SA that made him think that they didn’t ‘fit the picture’ gave him the tactical advantage when they made their move.