I've heard/read about people noticing things that others wouldn't, some of which I don't know if are true or not but once you notice things once, you can't unsee them.
Ones I can think of are-
businesses that are fronts for criminal activities, look for the same people going in and returning, rare delivery vans, not many customers, plain but opaque delivery bags (also notable for shops that sell illegal cigarettes!).
Random fireworks-not sure on this one but I have been told signify drug dealers receiving their 'loot' or gangs achieving something against another (I am skeptical about these ones)!
In city centres, sleeping bags in holes between bricks or narrow gaps between buildings-homeless people leaving them there to return to later. I never noticed this until I went to help someone look for her daughter who was sleeping on the streets :( she showed me lots of other little signs of homeless communities too.
I am ashamed to say that it was relatively recently I was told about the cobbled bits at the edges of paths and the textured pavements near roads being there so people who can't see well know when they're near an edge/road.
Do you have any?