Camera::Project transforms a position in 3D space into a 2D coordinate on screen, with a third component for the Z axis.
The coordinate does not necessarily have to be within the camera frustum. It can be outside the field of view, behind the camera, or beyond the camera's far range.
If the z component of the returned value is less than zero (or less than the camera near range, to be more precise) then you can skip 2D drawing. You can also skip drawing if the returned coordinate is far enough outside of the screen rectangle that the object can't be seen.