From season 2 onwards, we started seeing horizontal lights as well as vertical ones through the turbolift "window". Personally, I don't think that this truly IS a window, since the number of lights that pass in any given journey is very inconsistent even for similar journeys. More likely, it is simply a visual display for the occupants to give a vague indication of which direction the lift is headed. In s1 this is just up or down, but from s2 a hardware upgrade (in-universe) allowed the display to be slightly more informative, showing the occupants if they were travelling horizontally too.