Stranger danger isn't just a concept that applies to children talking to weird adults, it can apply to any of us at any moment. So, while the majority of strangers are harmless people just trying to live their lives, it's still smart to use a bit of caution when interacting with someone for the first time.
As a rule, it's always wise to trust any sinking gut feelings around strangers, even if they haven't given you an obvious reason for that feeling. In most cases, your body is giving you information your conscious mind hasn't yet received.
I was once driving in the country and is was pouring rain in the dead of night. So hard that I was driving very slowly and couldn’t see any of the signs (before GPS). I mean I had never been in rain this heavy. I had to pull over to the side of the road to try and see the sign to see if I was going in the right direction. I didn’t get out due to the rain so was craning my neck forward to try and make out where I was when suddenly the passenger door opened and a man got inside the car, sat down and looked at me. Here in South Africa, there is a fair amount of violent crime and hijackings, so I instinctively started panicking and yelling and screaming at the top of my lungs, flailing my arms to try and do something, anything to stop this guy from attacking me.