When he had been younger, Harry had dreamed and dreamed of some unknown relation coming to take him away, but it had never happened; the Dursleys were his only family. Yet sometimes he thought (or maybe hoped) that strangers in the street seemed to know him. Very strange strangers they were, too. A tiny man in a violet top hat had bowed to him once while out shopping with Aunt Petunia and Dudley. After asking Harry furiously if he knew the man, Aunt Petunia had rushed them out of the shop without buying anything. A wild-looking old woman dressed all in green had waved merrily at him once on a bus. A bald man in a very long purple coat had actually shaken his hand in the street the other day and then walked away without a word. The weirdest thing about all these people was the way they seemed to vanish the second Harry tried to get a closer look.
Why in the world would those wizards vanish? As far as I'm aware, there is no spell to disguise a wizard as a Muggle or that makes them disappear when looked at by Muggles. (We only see such spells on buildings.) And if it's Apparation, why would they all disappear the second Harry tried to get a closer look? (Bit convenient, no?)
So what caused those wizards to vanish when Harry looked at them?