Maintenance Plan mystery

Last week I had interesting (or I can say weird ? ) case of Maintenance Plan. One of the server that we monitor has reported job failure – this was a part of maintenance plan. Looking at the job history and log file I came to know that this job was failing because of some database which wasn’t available – this database was dropped few days back, and, the maintenance plan has it’s reference stored somewhere. I have search around system tables etc. but nothing unusual or nothing helpful was available that indicates the source of this database.
Finally, I have edited the maintenance plan which resolved this case, here is what I did:

1. Alter the maintenance plan "some maintenance plan"
2. note down the databases name
3. altered it and selected all the system databases
4. saved the maintenance plan changes and exit
5. re-open the maintenance plan
6. re-selected those databases (from step 2)
7. saved the maintenance plan changes and exit
8. re-invoked the job
9. and, the job completed successfully

And, the mystery persist, can you guys suggest me if I miss something to refer/research ?

-- Hemantgiri S. Goswami (http://www.sql-server-citation.com )