Should there be a fastener in this hole? I get fuel weep on the tank when I put it on the side stand after (over)filling it.

usually the water drain
 
dismount the tank, remove the complete unit and check/clean all tubes

I think it is to let rain water out when the bike is outside. If you overfill then the fuel will come out somewhere, don't forget that petrol expands when it warms up.

It is to show you if you have the cap on backwards.
If you can see the hole, all good.
Now everyone go out and see who can't see the hole.
I bet there are a few.

That part of the cap sits just below the surface of the tank and is prone to getting water colecting there. The hole goes into a aluminium tube and through the tank to the base and exits where the 2 tubes come out on the right side under the tank the other tube is the breather. If you have fuel comming out the cap the breather may be blocked or the seal around the cap is weeping which is the more likely thing.
Undo the 4 screws and the cap assy lifts out to inspect the seal etc.

When I got my K which was secondhand the dealer had replaced the fuel filter and it was on the wrong way round and the hole was blocked.
Regards,
K-BIKE