With TTS enabled, and when a Unity subscriber recieves a new e-mail, that should not enable MWI. Only an un-read voice mail will make the MWI go on, and it only takes one un-read voice mail to keep it on.
If in this situation again, we'd probably want to gather some traces. This doc can help...
http://www.cisco.com/warp/public/788/AVVID/Unity_MWI_Exchange.html
What it sounds like is that there really is an un-read voice mail left some where. To validate that, there's a filter that can be run on the user's mailbox in DohPropTest.exe. Let me know if you're interested in that.