I reckon you have a visiting slug or snail, lurking somewhere nearby.

That's what I thought. Sometimes you can get a really small one that's pretty good at hiding away.

Thank you. That is the answer! We've had snail/slug trails in the conservatory, and I've not been able to find the little blighters!

We get them too. They come in on the cats feet or fur. then they hide and come out at night to eat the cats food.

Hi Owdboggy. Happy New Year to you and Mrs. Boggy.
I don't know how these little creatures got in--maybe on gardening boots.... I'd just like to catch them.

Maybe use iron phosphate granules? Also--maybe too obviously--pick up all the pots and look under them?

Place a lid from a jar upside down next to the pot and pour a little beer into it. Snail or slug you will find it in the beer sooner or later. This plant shows occasional visitations from the appearance of fresh and old bite marks.

That's a good suggestion. Thx, I'll do that immediately!
I have checked underneath, Tugbrethil, and there is no sign of slugs.