Let me try to clear up some of the Post Office stuff (I was a clerk for them through 2005). A registered letter is a very rare type of mail and is signed for along every step of the way by EVERYONE who takes possession of it. Certified mail is what is used 99+% of the time. It is the geen label w/a green card you must sign that is attached, usually to the "flap" side of the envelope. This is only signed for by the carrier and the "accountables" clerk. Usually, only things of EXTREME confidentiality or items with great monetary value are registered (international mail is somewhat different due to mail theft overseas). The item is sealed with a specific tape over every visible seam or flap and is round dated (stamped) like a seal and initialed over those same seams so it would be apparent if someone tried to or did open the item, thereby breaking the seal. Certified mail is just a 'tracking" mechanism that legally demonstrates the delivery of an item. By regulation, the delivery must be attempted twice, no less than 5 days apart. After a total of 15 days, the item is to be returned to the sender with explanation, i.e. unclaimed, refused, etc.
A forwarding address does not seem to be the case here. But if one was filed as a PERMANENT change of address AND the recipient was listed on that change, it stays in effect for 12 months. Each of those items would be forwarded, unless the SENDER used certain endorsements that they pay for that would require the item be returned to them. Some are "Return Service Requested," "Address Service Requested," and others. Some of these I listed may not require the item to be returned, only that new address info be provided to the sender.
Finally, these accountable items are only tracked by the USPS by number, not by name or address. There is no way to go back in without a notice (it has the item number on it) that was left for you regarding the piece of mail to retrace or recoup the item. But with a notice it can be done but may take 3-6 weeks. Sorry about the length of explanation.