He could of deleted them, or the account got deactivated possibly. This is very simple if I want to know if ids are on the same account I goto the pw retrieval page, and I lock up the info on one of the ids, then I check the other ids, if the info on the other names is locked then they are all on the same account. I wouldn't suggest locking up a pw and using it to check it because it's not very consistent, if the info is locked, it works like clock work, and all aliases on the account will also be locked on the pw retrieval page, all it takes is 10 tries to lock up the info.
http://edit.europe.yahoo.com/config/eval_forgot_pw
I switched to the europe server, one day the default server wasn't working an I haven't used it since.