The zombie is a mere automaton. There is 'nothing that it is like' to be a zombie. Zombies behave just like we do, but are completely 'mindless' in the conscious sense. No one thinks that the existence of zombies who display no 'mark of zombiehood' is likely in this actual world. It is just that the existence of zombies who are behaviourally indistinguishable from us appears to be metaphysically, logically, and nomically possible.
Zombies sometimes bump into trees.
Zombies won't be able to do philosophy. These humanoid beings look like they see, harken like they hear, and act like they want. Jaron Lanier A zombie has a different philosophy. That is the only difference. Therefore, zombies can only be detected if they happen to be philosophers.