Why my character have used keys?

[Sir Du30]

Yesterday I found that my character has a few keys to open specific chests. Such as the Eatun Barn key, I found inside a cave at Eatun Barn. At first I thought it must be used somewhere, so I searched google about the chest location and it told me that there was a chest inside the cave where I collected the key but I looted it already. I guess it should be gone once it's used right? Now my character has a few keys which I don't know if any of them have been used or not. Can someone explain me about these keys?