I had the same thing happen to me a few weeks ago.  A case was opened and closed (buyer refunded) without me even realising it (my fault I guess for not checking my ebay messages, but I normally only check my emails and nothing came through via email.)

Customer claimed they never received their parcel.  I checked the tracking which said the parcel was signed for by the buyer by name and a copy of the signature as well but then at the same time the scanning said, wrong address, not known at this address.  It could be that the buyer changed their mind after signing for it, or the person who signed for it realised strait away it was not for her and gave it back and that they buyer had provided the wrong address (as yet it has not been returned back to me)

I disputed the claim, sent them proof of postage to the address supplied as they requested and today finally (it took about 2-3 weeks) I get a reply from ebay stating that they have granted my appeal and will refund me through paypal which will take 7-10 days.

Do you have proof of postage to the buyers address as Ebay specifically asked me to send them proof of postage to the buyers address (I sent them a copy of the shipping label and a screen shot from the couriers website also showing the buyers address).

Hope it goes well for you but it can take a couple of weeks or so.