Track Your Letter With QR Postage Stamps ?

Perhaps old news to citizens of Croatia, but as far back as 2011 the Croatian Postal Service started putting QR codes and a numeric tracking code to their stamps



You can then check to see how far your letter has travelled by entering the code on line

and check out its exciting journey on their interactive map

I'm not quite sure at what point the sender scans the QR code..after all, they have put the letter in the post box?