Hotmail and Gmail have for years added outgoing mail to the send folder as it passes through their outgoing SMTP server. I guess Yahoo are not doing the same thing,

It would be logical to have mail deleted off the server move to the deleted items folder. Again a similar approach is used by Hotmail, they move mail deleted using a mail client over pop to a pop mail folder in the deleted items. I know it is not what you are used to, but given the black box approach yahoo uses, it is a change I could see them making ithout any notification.