Our goldfish was recently found outside the pond, on the ground.
The cat was playing with it. The cat may have fished it out of the pond or the fish may have jumped out, as the pond was very full.
Is there a recommended level or gap between the top of a pond of tank the water should be filled to, to prevent fish from jumping out?
Fish that jump - will. Predators from land, in the water or in the air that hunt fish - will. The only way to ensure that fish stay in the area that is designated for them (pond or tank) is to put a cover on it, whether it be a lid, netting, screen.
You'll want to leave enough distance between a solid-cover and the top of the water to allow for oxygen (and other gas) exchange. This is typically about an inch (2.5cm).
It's also important that the cover not block out light so that plants can grow and fish still have a day/night cycle.
A bit off-topic, but important to note, is with a fish pond that is not above ground, it is very important to make sure there is a barrier or a path of lesser resistance for water run-off to not enter the pond. The biggest concern is pesticides or other toxic (to fish) compounds getting into the pond. (noted in the comments by @JohnCavan). You can also deter fish jumping (due to being startled) by providing cover such as floating plants (noted in comments by @MattS.) or underwater cliffs/plateaus, but you'll want to also provide a barrier so that birds and other predators are less likely to surprise the fish from above.
