In some way, Fortunato had insulted Montresor many times. The insult itself was never mentioned. The story begins with the reference that a thousand injuries (insults) had been borne but never had Montresor gotten his revenge for them. Montresor's family motto was such that anyone who insulted them would be dealt with. Montresor felt it was time to get the revenge for whatever the insults had been. So, he plans the murder to perfection.
A niche is an indentation in a surface. As such, part of the interior of the niche would not be visible. Plus, Fortunato was drunk and coughing from the niter on the walls. Probably his eyes were running as well. He had no reason to believe that Montresor was planning to kill him. I doubt that he was looking around for anything other than the cask of wine.