In fact, the superstar was singing the track live in a room next door, and the fans were left speechless when she appeared in front of them.
Carey later treated the devotees to a private concert to thank them for their continued support.
The event was filmed for the Late Night with Jimmy Fallon talk show and the singer couldn't help but smile as she watched footage of the surprise during her appearance on the program on Tuesday (12Nov13).
Carey credited Fallon for successfully pulling off the stunt, confessing she "was not feeling up to it", but admitted she had a great time interacting with her biggest supporters.
She said, "It's one of those things where it's just fun for me... they're right up this close to me. For me it was (one of the best moments of my life) too because I never get to do that. I don't even get to do that in front of my friends... Thank you for letting me do that because I didn't know what it was going to be (like)."