Mystery screw

This screw was found loose this morning on top of the inverter. Facing the engine compartment from the front of the car it was in the sort of trough area in the back left corner on top of the inverter. Just under the bit which holds the wipers.

Any idea what it might be for?

I have not done any work on top of the engine compartment in a very long time. However, I did have a mechanic change the brake fluid and try to figure out the cause of the extra 10 degrees F temperature on the front driver's side wheel several weeks ago. So it might be something brake related. It looks like there is a sort of rubber O-ring around the top of the head of the screw but that might just be carbon or dirt.

Thanks.

 

On a more serious note...

There aren't that many screws under the hood of a car, it is almost entirely bolts (and nuts). This looks like the sort of fastener that would attach something like a sun visor in the cabin, especially if it was accessed down a deep hole in a plastic panel. Or maybe an adjustment screw of some sort.

It appears to be a Phillips pan head machine screw, but no idea what diameter or thread. The black stuff that looks like rubber is wear or corrosion or something like that, it isn't soft.