Mystery Alley and Suspended Sand

This is Tokyo, Japan. I'm walking from my house to the park and I take a small alley with a wall on one side and expensive houses on the other. Over the wall is the park and the alley ends at the park which is where I intend to enter it. I know this alley, I recognize it but I don't know where it is. It is not on my normal route to the park or any other way I can think of. It must be somewhere else. Nevertheless I reach the park after going down the alley.
Among the expensive houses I pass a concrete designer house. The front door, located on the side of the house after going down an even narrower alley covered with tiny smooth stones, is very smooth steel, reflecting the house next to it. Farther down the stone alley is a staircase leading up to a second floor entrance. The staircase is made of a milky white plastic and is built right into the side of the building with no support posts beneath it. There are many such houses like this in Tokyo, I think, but still it gets hung up in my mind.
When I leave the park I go back down the alley. The entrance to the alley from the park is framed by the low hanging branches of an old tree. As I pass under these boughs I see the air is filled with motionless, suspended sand. I feel uneasy and push the sand from me and move forward into the alley. I don't want the sand in my hair or ears or down my shirt. I see a man in a navy blue sweater in the alley ahead just as he ducking out of it. I walk down to where he turned and find it is the stone alley. This is the second time the concrete house with the mirror door has presented itself to me and I think there must be something to it no matter how much I tell myself these kinds of houses are common and must belong to wealthy designers or business men with designer friends...from the 90s.
I knock on the door and a young boy answers.
Then I see the suspended sand under the tree again and a man's voice explains it to me. I think the man's voice may belong to the man in the navy blue sweater. Each grain of sand is suspended by a thread as thin as that which the spider weaves. It is a trap, but not for humans or other heavy creatures.