I once met a boy who was learning in Aish HaTorah who told me that he was upset by the fact that before he became Frum, when he passed Frum people on the street, they would say Good Shabbos to other Frum people but not to him (which led him to believe that Frum people hate not-Frum people). I pointed out to him that they probably didn’t realize he was Jewish since he wasn’t wearing a Yarmulka. He said that he was wearing a Magen David necklace, so they should have realized he was Jewish.
I didn’t point out (although I probably should have) that Frum people may not notice a Magen David necklace since it’s not what they are used to looking for!
In any case, the moral of the story is that when you see someone walking down the street, if you have reason to believe they are Jewish, say Good Shabbos to them – they are waiting for it!