It seems as if it disappeared. The Stockmann Outlet that was located at Keskuskatu is not visible anymore. But that doesn't mean it's gone.
Just a few weeks ago a new Nespresso shop opened its doors where the outlet used to be. But if you enter the new shop you will find steps leading down to another store.
And that store is the Stockmann Outlet. There is no sign outside giving any hint that you can still find it here. Apparently Stockmann doesn't really want any customers there. But check it out. You might find really good deals here.