In the Harry Potter books, you can only see a threstral if you've seen someone die, right? In the beginning of the fourth book (The Goblet of Fire), Harry sees "horseless carriages", but after seeing Cedric's death, he couldn't see them either. In the fifth book, however, (The Order of the Phoenix) he can see the threstrals. Why couldn't he see them in the first place, as he saw his mother (and several parts of Voldemort) die? Does it count, anyway, if you see a Horcrux "die"?