In the movie Wayne's World, Wayne and Garth are always avoiding Wayne's ex-girlfriend Stacy, so when she's coming they shout "Stacy Alert".
"Stacy Alert" is therefore a generic heads-up when a person you want to avoid is approaching.
Stacy: Well, don't you want to open your present?
Wayne: If it's a severed head I'm going to be very upset
Stacy: Open it.
Wayne: What is it?
Stacy: It's a gun rack.
Wayne: A gun rack... a gun rack. I don't even own *a* gun, let alone many guns that would necessitate an entire rack. What am I gonna do... with a gun rack?
Stacy: You don't like it? Fine. You know Wayne, if you're not careful, you're going to lose me.
Wayne: I lost you 2 months ago. We broke up. Are you mental? Get the net!
"UH OH! STACY ALERT!"