Hoster's

It looks like it. Lift up the pots and carefully look around the base and under the rim for slugs and snails. You can use slug pellets. These are now based on ferrous sulphate and don't harm birds, hedgehogs etc.

 

I had a good look but there are no snails or slugs to be seen anywhere, all I could see was one or two tiny little flying things, perhaps white fly by memory long ago!

 

I'm not quite sure if that's slug/ snail damage, unless the little critters were balancing on the pot edge underneath the leaf rather than slithering around leaving the tell tale slime path.



But something has definitely been in the middle of that leaf, under- or overside, having a meal without leaving a trace.

A catepillar perhaps?

It's strange that the edges of the leaf are fine.


*** edit***

After zooming in the photo, there seem to be some drip like marks on left bottom hand corner.

Have you been playing with any chemicals around the plant @potted? Any chance that anything got spilled on the plant?

 

I had a good long think, but the answer is no, nothing like that or anything else, there is no damage to any of the other plants next to or near it,I had a look at the little Hosta's but they have no damage to them either.

 

Here it is on the ground the day before, enlarged from a picture above.