A low frequency, high powered metal detector would treat concrete as simply more dirt and "see" through it. With a slab that nominally survived being moved around, the presumption would be it contains considerable reinforcing bar which is also going to be detected...so a skilled experienced operator would detect both the reinforcing grid and the freezer beneath. To a "hobby" detector, the reinforcing grid would represent a whole bunch of "hits" as squawks. The more sophisticated detectors would assemble an image on a screen.
The inherent premise here is that the bodies were to be found inside something metallic. If no prior knowledge of that was in place, LE would likely hire somebody in the ground penetrating radar business...looking for both anamolies in the excavation and the perimeter of the disturbed soil. Again an experienced operator could assemble an image of both the slab and the freezer, possibly even the bodies. GPR is frequently used to establish the condition of "old" CC as it can even detect larger cracks in a slab or beam, as well as undermined cavities in the fill beneath CC.
Of course the Misfits may or may not know any of this. The placement of the slab may have also been intended to prevent any detection of a sagged surface should the freezer collapse in future or to be simply a deterrence to any party that excavated that same area in future: much easier to bury a slab then it is to dig one up.
