well there really is no antonym that i know of for periscope because a periscope is an object.

someone discovered the periscope

a diagram of a periscope

Periscope is a noun.

As a "periscope" is an instrument for seeing above a surface of some kind (usually water) from below it, its use will typically occur in sentences dealing with military, exploration, or scientific subjects. Just one example would be, "The submarine was discovered by the scout plane through a fortuitous accident: the sunlight flashed off the glass lens of its raised periscope, revealing its location below the surface."