Trivia: In the opening credits there is a shot of a girl standing alone on the corner of a street. This was taken from the Buffy episode "Anne", when Buffy was staying in Los Angeles.
Trivia: By the series finale, Angel lost and regained his soul a total of three times: (1) in flashbacks to when he was sired and cursed, (2) after he slept with Buffy on her 17th birthday and re-ensouled by Willow, and (3) when Angel Investigations battled The Beast in season four (again re-ensouled by Willow). And while he didn't lose his soul in the episode "Eternity," he did briefly turn into Angelus; he also pretended to become Angelus in the Buffy episode "Enemies."
Answer: Everybody including Angel saw her. After helping them and then being left alone to talk to Angel, Cordelia says that the Powers That Be let her come back one more time to help them. After Angel gets a call saying that she died, he realises that it was her spirit that was helping them the entire time.