I was thinking the same thing too but how the cage works is by absorbing the radio waves and translates them into heat because the cage is self-short circuiting when the radio waves induce a current in the wire mesh. It may not be a perfect shield, but if buried far enough it'll evade even the best MD.
i just wonder if someone had the forethought to maybe do this to better hide their stash.
Well, that is not exactly how the Faraday cage works, but it's close enough.
The cage will protect its interior volume from an external, static --or slowly changing--
electrical field by setting up an equal but opposite electrical field on its surface. Thus whatever is inside, is protected from the external electrical influence. Note that the Faraday shield does not shield the insides from a dynamic or fast changing electrical field, or more importantly for our purposes, a slowly changing
magnetic field (ie. a magnetic compass inside a Faraday cage will still point North), which is what the detector uses to induce an Eddy current on the target.
The Faraday cage, being built of highly conductive material will produce a very large Eddy current in response to the detector's transmit field and therefore generate a very large detectable signal, most likely overwhelming any possible signals produced by the targets inside the cage.
During a lightning storm, the inside of an an automobile is one of the safest place to be if you are outdoors, precisely because it is a Faraday cage of sorts. The electrical charge from a lightning hit is confined to the outside surface of the car thus protecting the passengers.
Check out this MIT physics video lecture on Faraday shielding.