I was once in a K-Mart - in a time and place long before CCW was an option - when I noticed a group of 6 - 7 young guys walking through the store that just didn't look right. I told my wife I would be waiting in the car - where I had my 1911. Events after that proved that they were looking for "prey".
Playing it back in my head later I realized that what seemed wrong was the fact that they had walked by 3 rather attractive women without showing any signs of noticing them. I subconsciously recognized that they were looking for something other than merchandise.