I noticed the screw hole next to the glass lens is missing a screw. Why is the hole there in the first place?

It is a vent hole so air temperature equalizes faster, reducing the chance of fogging. Fogging can occur but if you use a anti fog lens cleaner before going into a harsh weather condition the moisture will bead and you will be able to see to center the dot. Don’t wipe the lens with your fingers, the oil on your hand will cause a problem. You can also wrap the lens with plastic and pluck it off before the shot.