This is not the best way to do it long term but, maybe it will help this once:
Get the date/time when the file was last checked in open the vault folder sort by modified date and find all files modified at that time.
If you have Security Auditing enabled you can pull up a report of items "Check In" by that user and find when the file was last checked in. This will help find the correct file.
Then you could do what Gary said and rename the files and open them up in the authoring application. If you have a lot of checkin activity then it may be hard to narrow it down to the exact file using this method.
However, I don't recommend accessing the vault directly you could accidently delete somthing!
You may want to concider removing the option to delete files for most users. I only like to give it to product/library administrators.