Waitress at the Window

There was a solitary diner in the restaurant- a garrulous, retired fellow who frequented the establishment during slow-paced breakfast hours and who spent his mealtimes attempting to make conversation.
The dining room had a single window that faced onto an alleyway and onto a great big wall of stone.
I glanced up during that morning and saw shadows streaking across the wall- as happened when somebody strolled through the alleyway.
But nobody passed by. I was thoroughly perplexed, and when I drew his attention to it so was the patron.
Neither of us could understand what we were witnessing: shadows- human figures- flitted back and forth, back and forth; but no persons appeared. On occasion the forms would raise their arms, and it seemed to me they were brandishing weapons.
We discussed the possibilities, which were limited to the presence of ghosts, perhaps spirits of diners past. The gentleman, naturally, could not sit and puzzle about it all the day long, but he was perturbed and intrigued by the matter. He requested that, should I ever find out the solution, I might let him know. I suspected that his routine and his days were not filled with sufficient mystery- thus the perpetual need for company while he ate, the eagerness to believe in something astonishing.
I found the answer shortly after lunch.
It consisted of no ghostly elements, but of repairs that needed to be done to the restaurant's roof, and of the hammer-wielding men that wandered around up there.