I found marble sized pebbles

[MikesToy]

While i was replacing my cabin air filter and cartridge, I found 6 little pebbles (size of toy marbles) inside that box where the circle-like area is. I couldn't see where the white box leads to but i could only feel that there's a wheel (kinda like roulette) with no holes downwards. I was worried i might have pebbles shooting at me thru the air vents. lol Anyways, i wonder where they came in from? I thought air would be coming from the top of the windshield but there's no way pebbles would get in there.

 

[jharris2]

The air intake is at the bottom of the windshield in the cowling. Something that size should not get through the screen.

Have any kids been in the truck where maybe they dropped the pebbles through the vents? Or, has an animal chewed through some of the ducts?