Coughing and Footsteps Behind a Locked Basement Door

I live in an urban-type settlement, basically a big village. A few multi-story buildings, a hundred single-story rural houses, and beyond that fields and little woods for hundreds of km around. Every evening I put the kids' bikes in the basement of a three-story building (our basement is like a shared storage for the residents of the entrance). The entrance to the basement ends with a staircase leading down about 3 meters into the basement itself - like in a classic horror movie, it's pretty creepy in there even during the day.

The other day we came back on the bikes from a ride very late, it was already dark as night (around 10-11 pm).

I walked up to the basement, opened the lock, was about to pull it out of the hasps, when I hear a child's cough behind the basement door. I think one of my little ones is coughing, I turn around - they're 10 meters away from me.

The little ones run up to me, I'm about to pull the lock out again - I hear someone coming up the basement stairs, walking up to the door and stopping.

The little one asks in a whisper - Who's walking there?

I'm in complete fucking shock and bewilderment myself, because it's already night, and kids have no business being in the basement at night. Plus the basement is a concrete box with one exit - up the stairs about three meters to the door. What kid in complete darkness, at night, locked in, would sit in the basement?
The basement of the building is divided into sections by reinforced concrete blocks without a single hole/door. You can't get in from the street, nor from the basement of the neighboring entrance.

The little one whispered "Let's drag the bikes up to the apartment for the night?"
That's what I did.

Both little ones didn't leave my side the whole evening, you could see they were really creeped out, even though they never were the fearful type.

Fuck knows what it was, honestly.

I heard the footsteps very clearly. The "someone-just-walked-by" option doesn't work, because everything around is perfectly visible - from the entrance to the building, the stairs in the building, to the space around the building