This saga has no end... After a few semi successful calls and emails with Apple, where I had to remind them several times to check the correct parcel number (they kept checking the Trade In Kit that arrived last week Monday), UPS finally updated its status to "Investigation opened"
yesterday - a full
10 days after the package disappeared.

Well today the status changed to "Service incident has occurred. / The parcel will be returned to the sender." 🥴I don't even know what to say anymore... Will phone Apple (again) tomorrow if I don't hear anything from them by then.
Until then I can only speculate what happened. They found it but it was damaged? They found it but the iPhone was missing from it? (They would not return an empty box though, would they?) They are returning
something, because the tracking says whatever they are sending is now back in Nuremberg, 2 hours away from me.
I'm guessing Apple now has to wait for the package to arrive to see what exactly happened before they can send me a new (or the old one again?!). To be continued.... 😒