Because landfills are now part of vanilla Factorio, your mod has been opened up to a bit of an exploit in multiplayer (and strictly multiplayer). If someone walks into a factory as the exit point, then another person picks up that factory and places it on an island, the player in the factory can come out on that island. This allows for a completely accessible and isolated system, which could be viewed as broken.
Judging from the way your code handles the factory being picked up with a player inside, I'm guessing you're storing the last valid position of the factory as the exit position. When it's placed again that becomes the new last valid location, allowing for this exploit of sorts. A potential fix (if it's worth doing) would be to store the location of where the player entered the factory instead, and use that position when exiting the factory.
Either way, just something I ran into, I'm not sure if it's worth fixing or not due to the specific nature of it. Thanks for making this great mod, and for taking the time to read this!
