If you fancy spending some money we have a product that can help with this, it could also save you mucho money on unused software licences
http://centrixsoftware.com
<\shameless plug>
And I am pretty sure you can find that out on SCCM
http://blogs.technet.com/b/breben/archive/2013/08/26/inventory-mapped-drives-in-configmgr-2012.aspx