There are four missiles in the storage building; not three, as the note indicates. The fourth one is sandwiched between a board and the upturned desk against the north-east wall, underneath one of the building's big trunks.
xbox360 When trying to exit after talking to Tabitha (if the player fixes the Rhonda before the quest is given), the door may be impossible to interact with, even though the "Open door" option appears. This is fixed only by reloading a save made before entering the building. This problem can also be solved simply by killing Rhonda. [verified]