And he glanced in the direction of the window.

Its narrow space was blocked by something.

And as he looked he heard a second object slither to the floor.
"Some one's dropping things down here through that ventilator," he conjectured.
And at the same moment a third fall sounded, followed almost at once by a fourth.

Then, for a second, the window space was clear, only to be blocked again as the person outside returned to his post.

And in quick succession three more objects were sent slithering down to the floor.