Terra ad Finem
Outpost #378-A is a place much like Area 51, in that everyone has heard of it but no one knows what really goes on there. No one even knows where it is. Popular theories include the following:
Outpost 378-A is a science station orbiting a black hole. This is quite clearly impossible, so the Ancients must have somehow made it happen on purpose.
Outpost 378-A is a Navy base in the middle of the Zhodani Consulate, just in case the Zhodani decide to attack the Imperium again. (Both the Navy and the Consulate have disparaged this notion, making it all the more likely.)
Outpost 378-A is the Ancients’ home world, which the Imperium discovered centuries ago and has hidden from view ever since.
Outpost 378-A is a super-secret intelligence base where bioweapons are manufactured for use against specific other races, such as the Vargr or the Aslan.