You must have a pretty big pond. I suppose they could have been hiding, they tend to find area's that provide shade, or a small cave. Do you have lots of moss in your tank? that too, is something they will hide amongst.
Fish are exceptionable at hiding themselves from predators. They have basically no defense to work with other than speed and hiding. Most fish hid during the day as this is when they are vulnerable to predators. They will wedge themselves anywhere they fit and can use anything to hid with.
Good news is if your fish have returned then most likely they were hiding deeper in their pond in a murky area where you could not see. The bad news, if they have not returned and do not for some time, most likely they have been eaten by a predator that lurks in your backyard.