I think the audience were given enough information. True, we weren't shown where the tunnel went, but we were shown that there was one. We could have at least suspected that it might have gone to the barn. And no, we didn't have the family tree, but we were told about the worker who was killed and that the girlfriend had been sending money to a disabled mother on another island. We could suspect these might prove relevant.
The give away was the murderer who seemed a quietly spoken sort of man was making all that unnecessary racket removing stuff from the shed and throwing it on the ground. I asked myself "why?" and decided he must have been the murderer before I knew "how?"
Pool seems to be modelled a bit on Mr Bean, though I think I said that about him in the first series,