Slugs and snails don't eat a hole through like that, they generally eat accross the leave and leave a track like trail that has the appearance of gouge marks. I'll post an image when I get home from work tonight.
Also, snails will munch on the edge of the leaves as well, and I can show some images of this too.
What I did was put the pot involved, in a container of water which was just above the level of the potting mix, but not high enough for the mix to float out of the pot.
Then left it for an hour or so, and as these criiters are not good at snorkelling, they decided to come up for air and I was able to find out what they were, how many, and at the same time, send them to their "Maker".
I had 3 miniature slugs doing the damage, but not any more.
What made the bore hole, I can't say, but I hope this remedy, may help you find out, as a lot of these criiters live in the potting mix and only come out at night.