You may not see any exposed screws....My guess is you can remove the interior finished skin inside the cabinet and expose a cleat at the wall where the installer at the factory will have used screws to attach the cabinet to the wall.
I learned this is the way mine is constructed when I removed a cabinet to cut it down in order to accommodate a larger TV. They use the void created with this construction as a wire chase when running electrical wires.
Hope that makes sense...if not PM me and I will try to explain better.
Thanks,
Phil