What to do? I found a engraved stone plaque at my yard!?

Call the previous owner. They probably had it transferred there when they moved from the last place they lived. They may have forgotten about it, until it was too late. Now they may be embarred to ask you for it. Call them and see if it was theirs, if it was ask them if they want it still. If it isn't theirs, then yes you should call the police.