Mormon 4

 As things got worse, while faithfully recording the history of the Nephites on the large plates of Nephi  (and maybe his abridged record), he shares - 

"...I, Mormon, seeing that the Lamanites were about to overthrow the land, therefore I did go to the hill Shim, and did take up all the records which Ammaron had hid up unto the Lord."  (Mormon 4:23)

Which in my mind, suggests that all the records did not remain in the hill Shim.  How long he had all of them before they were deposited in the hill Cumorah, I have no idea.