They do have lookouts but, in nighttime, if the tanker didn't have deck lighting on the lookout would only see the tanker's port/starboard running lights and masthead light(s). It's very difficult to determine range and bearing to a ship in those conditions. Radar and fire control will give you a much more accurate set of info on what the other ship is doing.
Do we know how dark or bright it's in that part of the world at 6AM at this time of the year?