The last place I rented was on a lake, and had a small 3/4 acre pond on the other side of the lake that only me and one other neighbor has access to. We would take a bass from the lake and "stock" them in the pond, which is fed by the same set of underground springs. After 3 years, we had a pretty good growth of bass. There's plenty of forage for the fish: frogs, newts, salamanders, dragonflies, etc., and I'm sure that some of the bass spawned in there.
Last year, however, it seems there were no bass to catch, only 3 inch baby bass. I'm not sure what happened to the adult bass, both of us practice c&r. The only thing I can think of is the couple of grey herons and snowy egretts that were hanging around ate them? I've asked this before, and people seemed to think that bass were too big for those birds, and muskrats (of which there were a few) don't eat bass. Also, there haven't been any dead fish floating in the pond, or on the bottom for that matter. Do racoons catch live fish? I didn't think they did, but now I'm not so sure..
I think it goes to show that fish can "leave" a pond, but they'll only get in through human efforts...