First time I've ever answered my own question!!
I posted on the Technic board, and it turns out that it is in-fact part of Better Dungeons as Dimtuhop suggested. These are apparently rare, but intended, cave systems.
Thread on Technic Forum.
So, for those interested, it's worth searching for! Just dig down to bedrock level and tunnel around. It's actually quite fun to explore and (although it appears to have the same amount of ore) there's lots of ore!
Using MCEdit, you can go back to your original spawn point by using "Move Spawn Point" on the hotbar and select "Goto Spawn". Then you can follow your history to try to figure out where your bed is.
If you really can't figure out, you can try the following MCEdit Filter. It will help you find your bed, but may actually take quite long (probably no less than a few minutes).
displayName = "Find beds"
inputs = (
("Search the whole world regardless of bounding box", True),
)
def perform(level, box, options):
if options["Search the whole world regardless of bounding box"]:
i = level.getAllChunkSlices()
else:
i = level.getChunkSlices(box)
print "Finding beds..."
for (chunk, slices, point) in i:
if blockExistInChunk(chunk, 26):
(x, z) = chunk.chunkPosition
print "=> Bed found near x={0}, z={1}".format(x * 16 + 8, z * 16 + 8)
print "Stopped finding beds."
def blockExistInChunk(chunk, blockId):
for a in chunk.Blocks:
for b in a:
for c in b:
if c == blockId:
return True
return False
Save the file under the filters directory in MCEdit with name findbeds.py, then go to MCEdit, use Filter on the hotbar, select Find Beds and click Filter. Now watch the console (the white on black window) and it will output the approximate coordinates of where beds are found.
Sample output:
Finding beds...
=> Bed found near x=-440, z=-600
Stopped finding beds.
Best Answer
The 5 I've found so far are these:
1: An item entity dropped near the Queen Ant. Not entirely sure if it was supposed to be like that or if that was a bug.
2: Inside the tree. Climb the jumping puzzle to the top and hit the button.
3: Inside the bed, in the house. Near the left leg at the foot of the bed is a set of stairs you can use to climb into the bed, which harbors "bedbug" spawners and cobwebs galore. Fortunately it gives you a sword at the entrance to this area.
4: Inside the bookcase, in the house. The forward-facing wall of the bookcase has a hole in it. Enter the hole, and complete the maze. Take the ladders up every time you find one.
5: Inside the pond. In the edge of the pond away from the house, underwater, there is a hole. Once you go through, the floor goes away, so hold space to keep your head pressed against the ceiling so you don't fall into the void. It looks like a dead end at first, but at the end go up, and you'll soon find yourself in a vast watery maze. There should be a quick path to the top where you will find a chest with a respiration III diamond helmet. Once you've grabbed that, make your way back down to the very bottom of the maze. On the floor is an area of glowstone. Break it and go down. In the chest is the pumpkin, and the button takes you back.
6:???