I'm surprised the enemy camps allowed such large objects to be brought in uninspected.
Absent of using an X-ray machine or actually tearing the boards apart, I suspect there wouldn't really be a great way to detect that sort of thing during an inspection. I'm sure they probably searched the boxes, but finding things inside of the actual board would be difficult unless you knew to expect that (then perhaps use a magnet to non-destructively locate files or whatnot cheaply? Maybe simply weighing them would suffice).