Shielding to Block Wi-Fi for Testing No Signal
Is this a common experience? Can anyone explain how this is possible?
  Faraday cage says what?


Copper has a skin depth of 1.3um at 2.4GHz, so it doesn't take much copper to attenuate a wifi signal. To properly build a faraday cage, you cant have apertures (holes), which you have some where the cables come out, or conductors crossing the boundary. 

The connection of the power lines is likely conducting emissions that are allowing RF to pass through the chassis. To adequately block these conducted emissions use a line filter. Line filters are special low pass filter that allows DC through the filter but has a capacitor around the outside (that connects to the chassis) to block RF. I'm mostly concerned about RF getting into the chassis, so I use them to severely attenuate RF (to a level that the sensitive analog electronics won't see it). They should also work well for keeping RF inside the box. 


Source: https://www.digikey.com/product-detail/en/tusonix-a-subsidiary-of-cts-electronic-components/4209-003LF/410-1088-ND/2079662

Here is a list of more of them. Select the approprate one for your attenuation needs at the frequency of interest.