Find the Water Thief

After some time has passed in your quest for the Water Chip, return to Vault 13 and talk to either Lyle (level 2, second room from the left of the screen on the south side), Cindy (level 2, first room from the right of the screen on the south side) or the water guard (level 3, outside the storeroom door) to hear complaints about a water thief. Hide out in the room nearest the elevator on level 3 and rest until midnight. When your screen comes back you should see a lone Vault dweller who will go into the water storage room (the room with the lockers). When he comes out of there, confront him and insist that he let you search him. Once you are done, you will recieve 1000 experience for finding the Water Thief.