Montresor had no particular reason to hide the stones and mortar, since nobody ever came that far into the catacombs. If Fortunato saw them he would not think anything of them except perhaps that there was some sort of repair work being done. The bones are on top of the stones and trough of mortar, but they may have been put there by Montresor to try to keep the mortar from drying out. The bones collect dripping moisture. After all, he did not know when he was going to be able to use it. He does not say that the materials were "hidden" or "concealed." This is how he describes them:
As I said these words I busied myself among the pile of bones of which I have before spoken. Throwing them aside, I soon uncovered a quantity of building stone and mortar. With these materials and with the aid of my trowel, I began vigorously to wall up the entrance of the niche.
Poe did not want to spend a lot of words describing how Montresor builds his wall. The author wants to get the job done as quickly as possible. That is why Montresor has the mortar already mixed. It would be a tedious and messy job to pour a whole bag of dry powder into a trough, mix it with water, and then stir it with a shovel. That is why the stones and mortar are all ready and waiting for Montresor. The trowel hasn't been left there because it would rust in all that dampness, so Montresor carries it under his cloak. Once he has Fortunato chained to the granite wall, it seems to take only minutes for Montresor to build the wall, even though he could not have had much experience in that sort of work. The climax comes when Montresor turns the key in the padlock. After that the story should end as quickly as possible, which it does. Montresor covers the whole wall-building, the conversation with Fortunato, and the passage of fifty years with impressive speed. Here is a sample of how quickly the wall goes up:
I had scarcely laid the first tier of the masonry when I discovered that the intoxication of Fortunato had in a great measure worn off. The earliest indication I had of this was a low moaning cry from the depth of the recess. It was not the cry of a drunken man. There was then a long and obstinate silence. I laid the second tier, and the third, and the fourth; and then I heard the furious vibrations of the chain. The noise lasted for several minutes, during which, that I might hearken to it with the more satisfaction, I ceased my labours and sat down upon the bones. When at last the clanking subsided, I resumed the trowel, and finished without interruption the fifth, the sixth, and the seventh tier. The wall was now nearly upon a level with my breast. I again paused, and holding the flambeaux over the mason-work, threw a few feeble rays upon the figure within.
He lays the first tier, then the second, third and fourth. Then he hastily lays the fifth, sixth and seventh all in this one paragraph. We can well imagine how much time and effort such a job would actually require. The mortar has to be troweled onto the top of a stone. Another stone has to be set in place. It has to be adjusted, pressed down, and excess mortar has to be scraped off. Poe intentionally makes the niche very narrow so that the wall will not have to be very wide to conceal it.
Within the wall thus exposed by the displacing of the bones, we perceived a still interior crypt or recess, in depth about four feet, in width three, in height six or seven.
The wall only has to be about three feet wide and six or seven feet high. The depth is only important because it has to be deep enough to keep Fortunato from reaching out and touching the wall, either while Montresor is building it or after Montresor has left and the mortar is still damp. Poe specifies that the chain holds Fortunato tightly against the granite so he could not reach out more than about three feet.
The quick answer to this is that the stone and the mortar that Montresor used to wall up Fortunato had been hidden under a big pile of bones.
Way down, at the most inaccessible part of Montresor's wine cellar was a bunch of human remains. They had been tossed down when the cellar was being made. The area had apparently previously been used as catacombs.
After Montresor walls Fortunato up, he piles the bones in front of the wall. No one will think to look behind them because it will look like it's always been that way.