Did you not go back to the ground? I went back and was told to go to the main reception and wait until the stewards had done their sweep of the terracing. All lost property would be there for collection once it was complete.
I had a happier ending as someone found my key. I was that overjoyed I was shaking everyone's hand I could see and slapping lots of backs...including Kris Commons, who I think I blanked when he said "Alright". Sorry Kris, I was just so relieved.
If they haven't found yours in the sweep then maybe they've been lost outside the ground. A fan would give the keys to a steward so maybe the last chance is to check that it wasn't a forgetful one who put them in his pocket and only realised later. Worth calling up again in a few days maybe?