Alien Supply
Once an Alien base is constructed, it is resupplied on a regular basis by special supply vessels. If one of these vessels is detected while landing then it is certain that an Alien base is nearby.
Aliens do not directly score for this mission.
Note that supply ships on supply missions are always at a speed that makes them impossible to shoot down by interceptors, even shortly before landing or after taking off. This means they can only be attacked while landed unless new aircrafts have been researched and built. Since they will certainly land this would be a waste of Elerium since your craft will need some for the interception and the Elerium in the ship might be destroyed.
Piracy time. You may want to let the Alien base exist, just for millions of dollars plundered from the Supply "Blimps". Arrrrrr, avast ye Space-Lubbers!
