i've implemented alien bases in my
Alien Bases branch on github.
it implements a new class called "AlienBase" containing all the variables (that i could think of) that an alien base would need. as well as a class "AlienBaseState" which is the popup that CAN happen at the end of each month to alert you of an unknown base. this should be a pixel-perfect reproduction of the original.
currently alien bases generate randomly on the geoscape within a certain distance of cities, but this at least puts them on the map until proper alien mission control is implemented.
please note that these bases spawn in UNDETECTED and require patrolling (or savegame manipulation) to discover, as per the original. locating them probably won't be easy, as they won't generate supply missions. they spawn NEAR cities in this implementation, so patrolling areas with high concentrations of cities (ie europe) would probably be your best bet.
detection SHOULD be at least similar to the original, with factors like distance and time spent patrolling taken into account, but i couldn't find hard numbers, so i can't be sure.
if anyone wants to try it out i have a pre-compiled copy of the executable
HEREany feedback/bug reports would be appreciated.