What can an octopus see that a vertebrate can't due to the blind spot in vertebrate (including human) eyes where octopuses have no blind spot? Where is the best place to hide a small recording device in a person's backpack (bag) so as not to be found? How? E.g. at the top-back or mid-back-side? Is there a "blind spot" in a bag that people don't tend to see or feel?