It's not every piece of first-class mail, it's only important documents. So they get their regular bills, letters from friends and family, etc. It's only important documents, the kind that you usually keep safe in a fireproof filing box, that go missing. The ones they specifically mentioned were their marriage certificate, birth certificate for the new baby, social security cards (multiple times) for the new baby, and baptismal certificate (really? how is this one useful to anyone?) for the new baby. So I don't think it could be a forwarding address issue, as other mail would go astray in that case. Otherwise that would be a good thought. They actually live in base housing for a military base, so they've taken it up with the base housing authorities and I believe they said they talked to somebody in the base commander's office, but they were as puzzled as our friends are and didn't know how to handle it. Oh, and they have one of those neighborhood lockboxes with a key, not a personal and unlocked mailbox, so it seems unlikely that somebody is stealing the mail at point of delivery. More likely it's either somebody at USPS or a distribution center, or something like that. But if that's the case, you'd wonder how the person would keep track of "oh, this looks like an official document for the Swarovskis, let me grab that," over such a long period of time, wouldn't you? So mysterious.
Somebody gave me a good idea in PM about why the SS card sent to a different address may have gone astray and how to fix it, so I messaged my friend to pass on that idea. They're probably going to be moving next year, but it would be nice if this could be resolved first! I wonder if a move would resolve it. It would *really* be weird if it kept up after they moved to a different state!