we simply don't know where it is. The assumption that lobsters are not sentient to any degree is only that - an assumption. And, when you get right down to it, there is not even a conclusive way of knowing that anyone, either than yourself, is actually sentient. We just do not know enough about the functioning of nervous systems to tell.

When, if ever, we come up with an explanation of exactly how a nervous system causes sentience, we will be in a better position to know.

BTW, in the 19th century IIRC there was a theory going around in certain European laboratories that animals such as dogs used in experiments were not sentient and therefore could not experience pain. So, they were strapped down and operated on without anesthesia along with other horrific experimental procedures.