I had a few more thoughts on this topic after I hit send:
1) In the sidebar, click on "All Items." Often, docs show up that do not show up when clicking "My Drive"
2) They can also search for the documents by name in the search bar at the top of the DRIVE page. The docs may show up for you.
3) Another thing to consider is that something happened while the doc was being synced to the cloud and the file got corrupted.
4) Shared docs also have a tendency to get deleted by one person without realizing that they're deleting it for everyone else too. Or someone might mean to hit Share but accidentally hit Delete instead
5) If they are shared documents inside a folder, and if a collaborator moves them, the docs may disappear form the owner's drive. In those cases the owner can still search for the file or click the All Items and they will be there. an owner moving/deleting/unsharing a document without telling others who had access to it can cause this.
6) In a school setting, one thing to make sure is that if the the computer goes offline while they are typing that they stop until it comes back online. Students often ignore the big red warning block and type away then wonder why everything is missing.
7) Another way that documents can vanish: Students failing to log out after using a school computer. Another student comes along and rather than logging out and then logging in, starts using the computer. They may not realize they are in another student's account, delete or move files. This is more common where students are not yet used to using accounts on shared computers,