@Accy cyclist.
Hi,I don't want to sound an alarmist or negative but thinking about it , that hole may of been drilled by the previous owner to let the water out because of an on going problem before you bought the car.Its also unusual to have rubber bungs underneath the car too, when you come to think about it.They usually have small holes in the floor pan when manufactured to allow the rust prevention to flow through the car shell when it gets dipped.There then plugged with a small circular plate of metal just like the one you can see in your photo (just behind the hole) These can often rot out with age.
I'm suspecting this hole you've spotted is supposed to be there though, to let water/moisture out of a box section.Looking on the dark side it's either been drilled to let moisture out of the footwell from an ongoing problem as there does seem to show some moisture around the hole.
I think your first thing to do tomorrow is to see if it's a normal drain hole or if you feel some sound deadening material/carpet behind it.