I never knew this until I read a forums post today, but Maintenance Plans have owners in SQL Server.
Beyond that, there is no way to change who the owner of a Maintenance Plan is in SQL Server Management Studio. Generally speaking when a DBA leaves a company, their domain user account is disabled in Active Directory, or at least I hope so. If that user account was used to create a Maintenance Plan, it will still be the owner, and by default the owner of the Maintenance Plan is also the owner of the SQL Agent Jobs that run the plan as well. The jobs can easily be updated to keep them running, but if the Maintenance Plan ever needs to be updated in the future, when the changes are saved, the owner for the SQL Agent Job is reset back to being the owner of the Maintenance Plan. A little research online, and I read more than once that you have to recreate the Maintenance Plan with a correct login to get around this, but I also found a blog post which shows how to change the owner of the plan with a simple update to MSDB.
SET ownersid = 0x01
WHERE name = 'MaintenancePlan'
The code is really simple, but only works for SQL Server 2005. In SQL Server 2008, a slight change is needed as noted in the workarounds posted on a connect feedback to change the behavior of Job ownership after editing a Maintenance Plan.
SET [ownersid] = SUSER_SID('sa')
WHERE [name] = 'MaintenancePlan'
If you find this interesting, please vote on the feedback as well.