A network share has permissions at the share level and the file system level. You state "user2" has been gone since 2012 but then you state there is no account in AD for user2. Let's make some distinctions.
user2 is an account name, not a person. The person who used the account user2 left the company in 2012, not the account. Anyone with the proper credentials for user2 can login. You can easily get lost in AD if you're not that familiar with it.
You didn't mention the server OS or version [Ex. Server 2012 R2]. However, if you have access to the server console, you can check to see if user2 has a session open.
From an elevated command prompt: net session
You can also check to see files open: net file
Ex.
C:\Users\Administrator>net session
Computer User name Client Type Opens Idle time
-------------------------------------------------------------------------------
\\192.168.0.113 LAZER Windows 2002 S... 4 04:54:05
The command completed successfully.
C:\Users\Administrator>net file
ID Path User name # Locks
-------------------------------------------------------------------------------
3556769893 D:\Users Shared Folders\engraver LAZER 0
4026532140 D:\Users Shared F...\4x2 UIL.cdr LAZER 0
4026532734 D:\Users Shared F...\lazer files LAZER 0
The command completed successfully.
If the file is locked, it will show the name of the file and the lock.
The session will tell you where the account is running from. It is always best to close an open/locked from from the app running it. If this is left over, you have other options.
From your command line: compmgmt.msc
Expand System Tools, Shared Folders, Open Files
Locate the file in the right pane, right click and select Close Open File
If the file will not close for some reason, you can always reboot the server. it will close all open files.
If after a reboot, the file reports it's locked, and it's not listed as an open file, then the app is using the file system to control locks to files.
At this point, knowing the app is going to be important. What you normally find in this case is the folder where the file is located, has a filename with a similar name but the extension is different.
Ex.
filename.ext
filename.lck
This can happen for many reasons. If an app uses the file system to lock a file for R/W mode and the file was not closed properly, then the file remains and you must manually remove the locked file. Note: This means only a particular app will be susceptible to this. Never work without a net, which means always backup files before tempting recovery.