See if ANYTHING else works with the key off that should only be working with the key on. Stuff like AC control, wipers, turn signals, etc. If ANYTHING else works, then I would lean towards what Sunlit said and look at the ignition switch. If you are 100℅ positive that it's only the windows that have power, then it's quite possible that the previous owner rewired the main power supply to the drivers side power window switch to a constant hot wire instead of the IGN switch. Also, when you are checking everything, be sure to check all 4 doors to see if the other window switches are operating or if it's just the driver's door. If it's just the driver's door then that makes me think even MORE that it was rewired.