Loose screws on ceiling fan screen

After traveling across the Sierras on I-80 in the truck lane (which is rough from chain damage in winter), I noticed that each ceiling fan had a screw rattling around on the screen.
No idea where they came from; couldn't see any empty screw holes.
Where could they have come from?

Rein

Loose screws, they are everywhere……. I know - No help.

Open the vents and check if the lift arm bracket and/or hinge is glued or screwed to the vent lid.

How’s the air quality around Tahoe?

We were in Clio's River Edge RV Park.  A bit warm but air quality was fine.

Rein