All salmon stray, wild fish too. A study I was involved in in Oregon many years ago, one in which tens of thousands of salmon scales were read to determine life history, demonstrated that the stray rate was about 3%. This makes perfect sense biologically, as if a river system got wiped out, like the Toutle after St. Helens, it would never repopulate with salmon if a few didn't try something new.

If you don't want hatchery/wild interactions, only breed hatchery fish that are incapable of spawning with wild fish timing wise.