Nighttime footsteps and the sound of falling ball bearings in a stairwell

Hey anons. I was reading about this shit where in panel buildings you hear a sound like the neighbors upstairs dropping an iron ball. And the neighbors might not even be home at all. It was supposedly explained that metal parts change temperature at night and make those sounds.

I've got a similar thing, but with its own quirks.
I live in a strange building, there are only two floors and 6 apartments. Only my apartment opens onto the corridor in the pic. The neighbor on my floor has his own entrance from outside, and the other 4 owls have to go up to their places through this corridor. By the footsteps you already know which neighbor is coming up or going down.
SO ANYWAY.
Sometimes at night I hear someone sneaking up the steps, slowly. And the fucker drops a whole bag of either gravel or ball bearings. You can hear the bearings bouncing right down the steps, hitting the walls. The first time I let it go, let him pick them up himself. And then it's quiet. Well I think, whatever, he's picking them up quietly. It's night after all, not the yard.
A couple weeks later, same thing, I think well fuck me, CLUMSY FUCKER, I'll go help pick up the bearings, and at the same time see what poor bastard is dropping them. Well as you guessed anon, there was nobody there. No poor bastard, no bearings.

That's when I freaked the fuck out, because the shuffling footsteps on the steps were clearly audible.

This shit has been going on for a couple years now, so I let it go, but still I'm curious what kind of FUCKER it could be.

P.S. and I've got two fighting dogs that react viciously to any human soul that approaches the house (except the neighbors of course, like I said only 6 owls live here). And when this shit happens the dogs have zero reaction to the footsteps, they only perk up their ears at the sound of the bearings themselves.
Even though when guests come to the neighbors and go up the steps, my doggies go apeshit in front of the door.
I asked all 4 owls, they said they didn't drop any bearings.