A scrap ninja is a junk metal collector who comes the evening before the garbage gets picked up but grabs the scrap metal, silently, and likely within the 5 minutes it took you to bring it out to the curb and walk back to your garage.
Wife: Did you take that aluminum awning out to the curb?
Husband: Yeah, like 2 minutes ago. Why?
Wife: <peering out the window at the curb> It's not there. Are you sure you took it out?
Husband: <also peering out the window at the curb> Scrap ninja must have grabbed it.