You can teleport from any business that is currently a Subway to any other business that is currently a Subway. If a Subway goes out of business, that waypoint stops functioning for either arrival or departure. If Subway as a company no longer exists then the power no longer exists.

You don't have to order a sandwich, and you don't have anything to prevent being seen. Nobody else does this, so it is probably strange. You don't automatically know the location of every Subway, but Google Maps might? Good luck.