I've heard it mentioned that offmapping is an exploit but I'm not sure what it is. I don't see it mentioned here in the exploit list /viewtopic.php?f=2&t=1053
Making a guess it could be building stuff off the map (I don't know whether that's even possible).
The reason I ask is that air units often go off the map too, especially if you are trying to get a transport around the edge of the map, I was wondering whether that might be something to do with it.