Me..
I was cleaning my bike after yesterday's damp-ish 200, and noticed the seat-pack was missing.
I'd just done a 200k into the remote wilds without any spare tube or anything.
The reason was that the last ride that bike had done was a wet 400, where I had swapped out the seat-pack for a bigger bag.
I'd removed the bigger bag when cleaning it up after the wet 400, but never replaced the seat-pack.
Idiot Boy.