Phyllis and Jim wrote:
Sounds as though they may be scared. Could you have had a predator
visit? Especially likely if they have been in the habit of coming to
you when you approach the pond (thinking you will feed them). If they
are scared, it may take a few days before they show themselves.

What is your water temp? They don't eat much if it is too cold...and
they will eat when it is still too cold for them to digest the food
well. I think most folk look for the mid-fifties before feeding.

Thanks folks. Rigged up a webcam and it appears that one of the
neighbourhood cats has been drinking the pondwater, maybe even hoping for a
snack as well as a drink!