She's leaving when you aren't paying attention.
When it is cold out, they may only leave for 15 minutes or so. When it is hot, they may leave for an hour or two. If you aren't out there all the time, you will miss the excursions.
It wouldn't hurt to put food and water by the dog house. It doesn't need to be IN the dog house because she'll come off the nest approximately once a day to defecate, eat, drink, stretch and dust bathe.
Is the doghouse in a predator proof area?