Before we added an explicit ID-Exists draft-iesg state, a decision was made (with IESG input) to have the expiration code move the iesg-state for an expired document to the Dead IESG state. Note that this could only happen if the draft was in an IESG state that did not prevent expiration.
Now that we have the ID-Exists state, is it a better target? Which will cause the IESG the least surprise (if a document moves out of "AD is watching" for example)?