By the time most of the party reached the stairs, the WHHRRRRR sound has stopped. You see Marius upstairs shooting an arrow somewhere east. The arrow hits the wall and you see not far from the point of impact a tiny construct sitting on the wall. It spins its little propeller, emitting the WHHRRRRR sound, and, clumsily, flies out of the window and then north.

Spoiler
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The party is mostly in S31T32 region downstairs. Initially, you can see the construct from there, and it can see you. Now the construct is out of sight of everyone except Marius somewhere AA23, at second floor height. Marius can see it through a window opening to Y25.

=> Party (everyone, any order OR you can just let it go)