I would say that the thing now is to approach people still, but perhaps not as aggressively.
"Hi, How are you, are you new to the neighborhood? I'm sorry, but I don't recognize you." Then you say, "What brings you here?" Etc.
And just be nosy in a friendly way. You'll figure out what's up based on their answers.
If they say, "oh, no, I live 6 blocks over, but my daughter needs to practice her bike riding, and your sidewalk is smoother than mine," that gives you info. And you can see what might be true.
You also can say things that make it clear that you DO know everyone, and that this is a generally alert neighborhood.
So if they say, "I'm visiting my friends over there," you can say, "Oh, I was just talking to them last week. They hadn't mentioned you were coming. How do you know them?" And listen for if they use the right names, or if their story just sounds about right.
A very friendly interest in someone who enters your neighborhood is always appropriate. And what you hear can help you.
As to whether it's rude or not--I think it's borderline "not done" (i.e., not "rude" the way we normally use that term, but "not good form," which is *also* the purview of etiquette).