Wifey and I like to go for a fast walk in our town along a busy street that has a wide sidewalk. At every street light post (about 30 paces) I look behind us to see if anyone is approaching from the rear. (My version of a Crazy Ivan!)
Several times, we've been behind other people and they NEVER look behind them. If we're walking faster than they are and we pass them, they have no idea anyone was behind them at all.
I know what you mean. I have seen people at gas stations with their nose in their phone and didn’t notice a stranger until they tapped them on the shoulder.
When a stranger gets within about ten feet of me I tell them to stop and if they don’t I then yell at them.
All the while with my Glock hot and ready.