Unless you go back to the exact same spot, you are unlikely to find that particular Matthias again. There are so many variables that finding the exact same Denpa man in two different places would be like trying to find two identical snowflakes.
You need a QR code, and unless someone else has been in that exact same spot and found the exact same Matthias, you'll probably have to make it yourself, as Chaosmaster suggested.
There's a topic here somewhere with a link to Caitsith's amazing program that let's you create your own QR codes. We'll post a URL for you...
Video games ruined my life! Fortunately, I still have 2 lives left.