The narrator is feeling overly confident at this point. He feels he has executed the perfect murder. The police have searched the entire house and found nothing amiss, and the narrator feels he's going to get away with the murder and the lies he's already told. He puts his chair right over the spot where the old man was buried as a symbol of his over-confidence; the police would never suspect he would deliberately sit over the spot he had hidden the body.
i think that the narrator is proud that he killed the old man and when the police searched the house they didn't discover what he did, so i think he sits there almost to brag at the fact that he got away with what he thought was a perfect murder
i think that the narrtor does that just to prove to himself that he is that good at killing people that the policeman don't even know about it
i belive it was to either, hide or test to see if the policemen would notice the recent alterations of the plaks on the floor.