do they have barbels? they are a koi if so. It is possible for birds etc to transport eggs from one area to another so it is not an impossible option

If you have fish in your pond that you didn't put there, then the most likely way they got there is either plants you may have put in your pond that had eggs attached, or someone else put some fish in your pond without your knowledge or permission. There is also a very remote possibility that some other animal or bird accidentally carried some fish eggs (on it's feet or paws) from another, not too far off, body of water.
However, before you start making too many assumptions you should positively identify what, the fry you are finding, really are.
When fry are really tiny they are very hard to tell apart, but as they get bigger it becomes a little easier. You say they appear to you to be tiny trout or bass. I think it would be easier to tell the difference between small bass and trout, then it would between small bass and small koi. Are you absolutely sure they are not small koi?

As bmaquatics says, check for barbels (Little wiskers). Of course catfish have barbels too.