We currently have a problem when users open drawings in 2013 Autocad, then when a user tries to open in 2011 or 2012 then the file is becoming corrupt. Originally I thought it was due to the user not saving down but this is not the case, it is just being opened.
By corrupt I mean that there are things missing from the drawing when the person in 2012 opens it.
Missing is not Corrupt, please use the correct terms when seeking help: so what exactly is missing? describe in detail? Did the 2011/2012 users turn off all the warning prompts common to AutoCAD OOTB? These can be turned back on in OPTIONS command's SYSTEM Tab, General Options Section, HIDDEN MESSAGES SETTINGS button.
AutoCAD loves to tell a user what's wrong when they first open a file, let it do so.
FYI: 2013 is not only a new DWG format, it will upgrade any AEC content in a DWG file to a new format. So when saved down, these become PROXYOBJECTS and if the lower AutoCAD user turned PROXYNOTICE off as well as all the other PROXY variables, that may be part of the problem. Plus 2013 adds it's own new objects and does the same.
A common fix is for the R2013 user to use -EXPORTTOAUTOCAD command to knock a file down, not SAVEAS or a SAVE setting change on OPTIONS.
If you have no idea how to diagnoze a file as I suggested above, please feel free to post such a 2013 DWG file here (delete any sensitive info), give instructions about what may be "missing", and someone here can open that file and dissect it for you. We all have 2013 and 2012/2011, don't be shy.
I know you have stated in your original post that "it is just being opened", which makes it sound as if they are not saving the file, but in order for changes to occur as you say, they must be saving the file before or as they close it.