If this is a version after Ac2007, user-level security has been removed so there is no way that would be the culprit. That means that there is something about the way you are storing the record such that when you try to retrieve it, it is being filtered out.
In no particular order:
1. When the user stores data, is there a record of who stored it?
1.a. IF the answer to 1 is YES then does the retrieval method (query or form's .RecordSource) CARE who stored it?
Yes, one of the field is their environ username. It is important who stored the data
2. When you say that the other users cannot see what was stored, is that "EVER" or merely until the unusual user exits the app?
Even after the user exits the app, other PCs still cannot see his data.
2.a. IF the answer to 2 is "Visible after exit" then does your data storage include use of transactions?
3. When the person stores data, can the same person exit the app, then re-open it and see what was just entered? (Even though others cannot.)
Yes
4. Just for clarity, remind us what version of Access you are using. That question applies to be the FE and BE files separately, since they COULD theoretically be created and maintained by different versions of Access.