If computer disks disappear and no one is there to see them do they exist?

Am having the Monday From Hell, being very far behind and exhausted due to insomnia. Nonetheless, have to stop and comment, again, on the flap last summer over the "missing" computer discs at Los Alamos National Laboratory. Have already made fun of this in 7 days, but Saturday's Journal North carried an AP story out of DC in which members of Congress learned that it cost $367 million to close the lab last summer.
"Lab officials virtually shut down the facility last July after reports that two classified computer disks had disappeared. An investigation later determined they never existed."
Some parts of the lab didn't re-open until last month!
I find this story continues to amaze me. Did they reallly not exist? What was allegedly on the non-existent disks that was considered so important they had to shut the lab down? How do you determine something doesn't exist anyway? Isn't that proving a negative and isn't that supposed to be impossible?