Since the Spring Seller Update (2014), eBay counts every case that is opened for "Item Not Received" or "Item Not as Described" as a defect, regardless of the ultimate outcome (i.e. regardless of whether the item arrives, in the case of INR, or the seller refunds, in the case of INAD).
Compounding this issue of course is the problem of buyers "accidentally" opening cases when all they really want from the seller is an update. EBay has changed some of the buyer experience in this regard, but in my view the communication process is still not separated sufficiently from the case opening flow.
You can try calling eBay to get this removed if you have a lot of time to spare, but I think your chances of having the defect removed aren't good. If you have any subsequent messages from the buyer saying that he opened the case in error, that could help.
EBay made a decision in the spring of 2014 that anything that reflects buyer dissatisfaction, even if only temporary, and even if cheerfully and promptly remedied by the seller, is worth a defect. Their basis for this thinking was that buyer dissatisfaction would mean buyers would not return to eBay to buy.
Whether eBay was right in supporting that principle in the way they have is debatable, but we're now stuck with it. This also means that eBay is now holding sellers responsible for some events that are totally out of our hands (such as delays in delivery of parcels).
For reference, here is the Q&A from the Update that pertains to opened cases, with the link to the 2014 Spring Seller Update below:
"Why is eBay counting opened cases, returns, and detailed seller ratings of 3 for item not as described?
The data shows that receiving an item that is not what the buyer expected based on the seller's listing is the single greatest cause of a buyer deciding not to purchase again or purchase more often on eBay.
What is considered an opened case?
