From Dicicco, When opening a document in the file area the last user column does not change. It doesn't change the name or date. It stays as how it was.
Helpdesk:
The developers have considered this as an Enhancement.
The only way the column " last user" is updated is when the document is edited or modified. Opening the document does not trigger anything in code to make a change. If you want to find who open the document last that can be found in the document history page. The developers do not consider this a bug and have put this as an Enhancement request. All this means is this does not have high priority like a bug and will take some time to make the changes in code for this reqeust.