Wow! Is it possible that someone released it there after picking it up and bringing it back from its natural range? Or maybe it escaped from someone nearby who had been authorized to adopt it as a rescued animal.
Did you leave it in the park? It might be a good idea to call the CA Fish and Game Dept. or whatever state agency regulates the rescues of Desert Tortoises and report the sighting so it can be collected legally before something bad happens to it.