The Great Garden Mystery, AKA, The Case of the Missing Fish

[Earlybird]

Unsure whether this belongs in 'Gardening' topic or 'Pets', but here goes....

We have a small but fairly deep (waist deep?) ornamental pond in the garden. There are about 18 fish in the pond - with the two largest being young white/golden koi (both fat, and maybe as long as a man's hand). For the last two days, I have not seen one of the koi. It is not in the skimmer box/filter.

What could have happened to it? Should I worry about the remaining fish?

If it's def not there could be cat or(most likely) a heron.Could anyone have got into your garden as I have heard of people stealing fish from ponds especially lage koi as sell for quite a lot for fish.
Hope it's none of these and it's just hiding very well.

My cat once came home with a fish out of someones pond- i never admitted to it

Herons often steal fish from garden ponds.

We had visits from a heron around Christmas (but not since then), but the pond was covered by a net so fish were safe. I had the pond professionally cleaned about a month ago, and the fellow said he didn't think a heron would be successful in an 'attack' here as there is no shallow place for the bird to stand. He thought a net was not needed, and said this wasn't the season for herons to go wandering anyway as there is currently plenty of food in their usual habitats.

A cat is possible, I suppose - but garden is entirely fenced, and as I said before, there is no shallow water. But I have on occasion seen cats who simply scale the wooden fence - so maybe one was ingenious or lucky?

I keep thinking that perhaps the fish got stuck in the rocks, or got ill and died. But there is no trace of him/her so far.