On second thought, after reading these comments, it’s probably an ozone plate. BUT - the other silver plugs could be in the ozonater. I still like your way of thinking - who knows what people will do?
I watch too many British mysteries.
People have been hiding their treasure since prehistory. It’s not a bad idea, really, as long as those you care about know where it is hidden.
I had a Spanish friend whose family still had a hacienda in Mexico. While cleaning out an old water well, they discovered some displaced stones, behind which where the armor of a Conquistador, along with some other valuables. Needless to say, they did not inform the authorities.