A duranium shadow is a form of phenomena, methodology or technology that can give off false sensor readings.
In a confrontation with two Cardassian warships shortly after the discovery of the Bajoran wormhole, Major Kira Nerys and starbase Deep Space 9 used a combination of thoron fields and duranium shadows to give the appearance that the space station was bristling with armaments. (DS9: "Emissary")
In the Klingon confrontation on Deep Space 9 in 2372, a Changeling impersonating General Martok charged that the station was using these same two tactics to give the appearance that the station was heavily armed; unfortunately for him, that hypothesis proved to be quite false. (DS9: "The Way of the Warrior")