Light IS still being reflected/bounced by the body of the invisible object before hitting anything inside...
The light that is reflected by the body of the invisible subject is the light that would have been reflected back to the observer if the subject would not have been there. The spell mimics the surroundings from all angels.
SR5 p. 291 Improved Invisibility
Improved Invisibility mimics the subject’s surroundings from all angles.This can be read as if it mimics the
inside of the van (flying shadowrunners and visible flying grenades in the bag) or that it mimics the
outside of the other side of the van (invisible shadowrunners and invisible grenades).
However, the whole question is moot since you can't use Improved Invisibility on a drone or vehicle just as little as you can't use Physical Mask on a drone or vehicle. To use mask on a drone or vehicle you need the Vehicle Mask spell... And there is currently no Vehicle Invisibility spell.
IF they add a Vehicle Invisibility spell in the future THEN I am sure this spell will explicitly specify that it i will mimic the
outside surroundings from all angels, making everything inside the drone or vehicle invisible as well.
If you rule that you can use the current invisibility spell on objekts you also need to classify where the limit is drawn. Can you make a drone invisible? What about a van? A bus? A helicopter? A ship? A passenger plane? Can you use it on other objects.... A table? A weapon? A bag with grenades? A house? Why stop there.... what about a skyscraper?