Some papers were sent back to Britain for safekeeping, per the article. Speculatively, perhaps someone who was there could also tell us what was burnt in more detail.

It's the second line of the article...

"A great number of these records were destroyed and a sizeable portion sent to Britain to be kept secret. "

Not all have been destroyed.

If document-A makes a reference to document-B, and document-B is missing, and there were other references to burning, that's a start.

Also, document-B might show up in the set that wasn't burned, but taken away and classified

Maybe they wrote down what they burned.