External UFO Windows
It is sometimes possible to peer inside UFOs via windows set into the external hull. It is usually necessary to be at a different elevation from the level being viewed. Flying Suits are excellent for this purpose, although hillsides or buildings can sometimes be utilized.
The exact angles/positions required for viewing are not known at this time.
This phenomenon may simply be a result of the game engine failing to properly calculate what should and shouldn't be visible from certain positions. Case in point: There are some situations where no UFO windows are apparent, but you are still able to see into the UFO.
Further research is needed.