My Grandmother is a Holocaust survivor who was sent to Płaszów in 1943, at the age of 14. At some point, the Nazi’s decided to confiscate any remaining jewelry in the prisoner’s possession. When she was 9, my Grandmother’s beloved cousin Salek gave her a delicate sterling silver dachshund necklace. Instead of allowing it to fall into the hands of the German’s, my Grandmother buried it next to her barracks.
This story has stuck with me throughout the years. I know that Płaszów was dismantled as the war was ending and the area is now mostly hills and fields. I can’t help but wonder if my Grandmother’s necklace might have been found over the years or could still be there where her barrack was. She is almost 92 years old but still more than with it. I know looking for her necklace is basically a needle in a haystack but even needles get found on occasion.
This one is a difficult one since unlike, say, Auschwitz, there isn't an institutionalized Plaszow memorial. Assuming it was found and assuming the finder didn't just keep it, there are several possibilities, including the Auschwitz memorial and the Polish Academy of Science.
I would recommend contacting those two institutions about objects from Plaszow and specifically that necklace because they can at least give you a sense of where something like this could have ended up if it was handed in at all (e.g. if there indeed were official archaeological digs at Plaszow) resp. on how to proceed should you want to go look for it in person.
So either try contact@holocaustresearch.pl for the Academy of Science's Holocaust Research Center or someone from the Archives or Research Section at the Auschwitz Museum.
Best of luck!