A single user profile keeps getting removed from UPS

We have a client using SharePoint 2010 with SP2 installed. We have seen over the past week that one user's profile has been removed twice from the user profile service. As far as we know, no changes have been made in AD to the user's account. We have also checked FIM and saw that the last time the profile has been updated is 6 months ago. Why would a profile be removed from the UPS?