According to the chairman of the Senate Intelligence Committee, "If that is in fact what happened, the counter-intelligence implications have increased by an order of magnitude. We may have someone on the inside who doesn't want anyone to know they had the drives or what they did with them . . . ."
If someone really did take them out of the lab, I'm a little confused as to why they'd bother trying to sneak them back in. Seems a little risky, compared to simply letting them stay lost forever (since whoever it was obviously didn't get caught taking them in the first place).
The best I can come up with is that they wanted to make it look like the drives were there the whole time, so all the FBI agents would calm down and go home. But get real; the drives were found stuffed behind a copy machine. What's the line of thought there? "Oh, gee, big mix-up. Apparently somebody was trying to photocopy the drives, and the lid flew up and they just fell back there. Woops!" It'll be interesting to see if they ever find the person that did this. At the moment, it's not looking good.