[mnehring]

It looks like a drywall repair patch. It glues to a place where you have a hole in the wall, and the mesh holds putty.

[yefragetuwrabrumuy]

Put two and two together. If he had something of value stored with something used to patch drywall, there is a good chance that he stored something of value inside of drywall, then patched it to look like normal.

I would guess a whole bunch more silver plugs.

It sounds like time to get a metal detector.