Lots of things, including the terrestrial planets, have a lot of metal, including in the crust, which on Earth is mostly silicon and aluminum. IOW, uh, no. It’s an old speculation, and even Sagan suggested it (as his own idea?), but there’s literally zero evidence for it that isn’t also evidence for a prosaic, mundane origin.
It’s an ill-kept secret that Russian penetrating radar has shown rectangular interior spaces.