I run a new pixelmon server starting with a fresh world recently and we render a map of the world using Overviewer so this can be viewed in a browser. I have noticed single, isolated chunks scattered across the -Z, +Z / South-West quadrant of the world outside of what has been discovered or generated by Minecraft. I believe these to coincide with the spawning of legendary pokemon. This is only in the South-West quadrant of the map and these single chunks extend out to about -50000 on each axis. We certainly haven't discovered that far out! These single chunks are in the region folder of the world (shortened list as it's hundreds of these):
(sizebytes regionfile)
5251072 r.-1.6.mca <--discovered map
12288 r.-190.0.mca <-- way way off the discovered world!
12288 r.-190.14.mca
12288 r.-170.-7.mca
12288 r.-172.-32.mca
12288 r.-173.4.mca
12288 r.-174.-8.mca
.
.
12288 r.-57.35.mca
12288 r.-57.45.mca
.
.
12288 r.-25.15.mca
12288 r.-25.21.mca
These can be viewed on the online map at
http://ob-mc.net/pixelmon. Scroll over the south-west area and zoom in to see the single chunks. I couldn't see any in other quadrants, so it looks like the spawning algorithm has some kind of bias to that area and extending out very far.