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.
Underneath - S5-E17
Trivia: In the episode "Underneath", Illyria is talking about different dimensions she has visited and she says "And one world with nothing but shrimp...I tired of that one quickly." This is a reference to Buffy episode "Superstar": when Anya is explaining that there are many types of alternate realities, she says, "And there could even be a world with nothing but shrimp."
Inside Out - S4-E17
Trivia: Skip's line in 'Inside Out' - 'Nobody comes back from paradise. Well, maybe a Slayer once,' is a reference to BTVS, and Buffy being brought back from heaven in the episode 'Bargaining'.
Trivia: In the episode "Orpheus," the actor who played the armed robber that shoots the cashier in the diner is played by Nate Dushku. He's the older brother of Eliza Dushku who also appears in the episode as Faith.
Hell Bound - S5-E4
Trivia: Angel leaves the Reaper locked in a metal cage as the camera pans slowly back so we can only see his eyes - almost exactly the same shot as when Connor left Angel trapped in the metal coffin at the end of "Tomorrow". They even say the same thing - "You get to live. Forever". Like father, like son.
Trivia: In the episode 'Untouched' Angel asks Cordelia 'Do you have any idea how hard it is to think when you've got a rebar through you?', her reply of 'Actually, I do. Benefits of a Sunnydale education' is a reference to the BTVS episode 'Lover's Walk', in which Cordy falls through a staircase and is impaled on a rebar.
Trivia: In this episode, Cordelia makes a reference to "self-destructing tapes and a dossier". This is a reference to the 1966 TV series Mission Impossible, which featured self-destructing tapes used to send the main character on his assignment. An interesting fact is that Mission Impossible regulars Martin Landau and Barbara Bain are the real life parents of Juliet Landau, who played Spike's former love Drusilla during season 2 of Buffy.
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."
Carpe Noctem - S3-E4
Trivia: The episode title is a play-on-words of the Latin phrase "Carpe diem," which means "Seize the day." "Carpe Noctem" literally means "Seize the night."
Are You Now Or Have You Ever Been - S2-E2
Trivia: When Angel is outside the observatory, he is dressed in the same clothes that James Dean wore in Rebel Without a Cause (a film where there are significant scenes at the same observatory), and Judy has a bit of the Natalie Woods about her too. They are also both set in the 50s.
Blood Money - S2-E12
Trivia: When the Special Guest TV Stars come out and "rob" guests of their donations at the ball, one of the starring ladies is asked "What's the deal with making your character a lesbian? Is it for ratings?" A little jab at Willow's coming out on "Buffy the Vampire Slayer"
Trivia: Joss Whedon hints at future events in season 4 during Lindsay's song in "Dead End." It hints about the coming of Jasmine: "The sky's gonna open, People gonna pray and crawl, Gonna rain down fire, Gonna burn us all," and "Pretty as a picture, She is like a golden dream." All references to Jasmine.
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.