The cat is out of the bag. If the coin were to disappear at this point, the government would probably sue Mr. Lawrence for the appraised value under a theory of conversion. It was in his possession and now it isn’t. The fact that he can’t find it, does not mean he did not owe a duty to preserve it until the ownership issue is resolved.
Big deal. Let them prove the worth of something not there. Worse comes to worse, I would just manufacture a new "penny" from a mold and pour molten aluminum into it to create one and let them find it on one of their searches. I once made a giant aluminum "nickel" about nine inches in diameter as a project many years ago. It is not difficult.