This exploit is a bit useless, I also can't really give you much info about it yet.
This happened to my brother while he was playing yesterday, he was using the FP to another city and it happened to pass trough WG right when it was about to end, making him earn one free WG mark. Making this simple you don't need to join WG to earn marks, you just have to be inside the map, using the FP or trying with the flying mount.
I do know this is hard to remake and also I don't have any "proofs" ~ so just try it if you want, if your able to make it try to take a screenshot for me
[Don't forget you DON'T have to rep since this isn't much usefull information, but make sure to remember that you don't have to flame like one idiot because of it.]