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.