I sent it from Meppen (Germany) to Gabriele in Trier (also Germany) in 2005. And apparently the card couldn’t be delivered (“recipient could not be found”). I have no idea how it took the German mail more than 15 years to get it back to me, my address is on the card and it’s the SAME COUNTRY
Maybe a daily newspaper in Trier would be willing to write a small article about it and maybe you’ll be lucky and the recipient will read it and contact the newspaper or you.
But before that, you should ask the support if it is allowed to mention the recipient’s username in the article.
And maybe you’ll feel like writing some cards again.
Yes, I did put my address on it just in case, for situations like this.
I’ve sent over 400 cards through postcrossing and only a few came back, none of them took this long though.
Fortunately you put your adress on it, so at least you got it back. I ususally don’t put my adress on postcards, maybe I should from now on, because recently a large amount of cards got lost - or will arrive in 15 years