Someone has been in the hatch before?

Ive read somewhere that someone has already been in the hatch before.
There are also some speculations that Kate might be the one that has been in there.

Any ideas who, why and how?

My theory is that it is a game show and the people who died lost in the game. It is a strategic game and one which call on the survivors to survive, obviously. However the fact that someone has now opened the hatch and they've found a computer gives the game a whole new direction. The survivors now have to have staminer to sit at the computer and keep typing in numbers. Are the numbers going to be the date of births of the surviving contestants?

I would be interrest to hear how anybody (without TNT) could have been in the Hatch.

John Locke - A bright light - S1 E20 and following

well remember anyone could have gotten into the swan without venturing down the hatch. anyone could have found the back door and entered whenever.

This was just a rumour.. .. i guess if there was already someone in there we would have known by now..