Did you check favebook to see if she has an account?
It would not be too unusual for a full time nomadic dweller to chznge private mailbox comoznies.
One thing you might not reslize is that you cannot get a forwarding address from the US ppostal service for s private mailbox. That it is strictly up to the private mailbox service to forward all the mail. I know this through having applied to hae the USPS to forward mail from my old private mailbox address to a different private mailbox. The USPS does not accept forwarding for mail that goes to businesses, it only does that for residential addresses. It is strictly up to the business to put on a forwarding address on the mail and then place it back into the mail system.
When I filled out the USPS mail forwarding request it was rejected with that explation included in their letter. That being said she might have gotten a new mail service and never have received the notice from the USPS thinking it had all been taken care of.
So just because your letter came back with "no forwarding address" is not a reason to panic. That notice you got was policy and not a surefire indication that she has actuallly gone missing or that harm has come to her.