The documents had value to Trump, for memoirs, souvenirs, evidence, proof, etc.
Classified documents can NOT be accepted by NARA.
The point of the National Archives is to make documents public, which does not include these.
They do not have to be stamped "declassified" because Trump can still access them if the are not declassified.
So there was no point in bothering.
Having top secret documents in a highly secure mansion, inside a highly secure upscale resort, is totally safe.
No body would kill to get what they do not know exists, in such a highly secure environment.
It likely would be easier to steal documents from the White House, where lots of unknown contractors regularly show up.
As to pieces being left, I had not heard of that?