Yup, the holes only arise when the larger anions are packed together in any style, however, this does not mean that they cannot be represented in a unit cell, for the holes are spaced at regular intervals too right?
As you can see in the second picture, all the white small circles represent the tetrahedral holes in the body centered arrangement, as they are formed by 4 main "spheres" arranged around it in a tetrahedral fashion, as represented by the bold black lines.
In the case of bcc, the octahedral holes would be right smack in the middle of the 4 tetrahedral holes lying on the top face, or at least that's how I see it..(octahedral hole created by the 4 corner "spheres" on the top face, and the 2 spheres on top and below it)
Hope I cleared up some confusion!