Just because an envelope indicates the letter was returned because the person was not at that address does not mean that they never lived there.

They just could not be could not be found there on that date. Use the address to look for the person in local records before the date of the postmark. They could have lived there for years before the letter was mailed. And, in the case of large cities with specific addresses, they simply could have moved a short distance away and left no forwarding address.

Even “errors” are clues.