Light coming through the lens is travelling about perpendicular to the film, it won't get far. If you open the mirror box, it comes in at lots of angles at much, much higher intensity and travels all over. Not to mention that window light for 10s without a lens is many thousands of times (10-15 stops!) more light than a correct exposure, so it's quite possible even for film with good anti-halation properties to pipe the light around for a bit.