There was a locket that "nobody could open", mentioned just in passing. I think it went to the rubbish bag, where Kreacher removed it.
That's my story and I'm sticking to it! I suggest they check his "den".
Wonder what's become of all of Kreatcher's things? Are house elfs allowed to take their belongings with them when they move from place to place? The locket might be at Hogwarts.