So I think I found the issue...
The content/admin and user, were opening an excel XLSX from the lirbrary, and attempting to save a single sheet from the XLSX back to the library as an HTM file.
When this happens, you have to give the "new"(HTM) document a Title. It a requiered field, no matter if you have document check in/out enabled or not. You can do this in the "save as" dialog box when saving the file back to the library.
But you CAN'T do it once you have uploaded the document.
Once the new HTM file was given a title, it did not show as "checked out" upon upload anymore.
And just for explanation sake, this was MOSS 2007 standard, and Office 2007.
Joshua Fuente Farm Administrator SharePoint Products & Technologies