Not related to the observed phenomena but maybe interesting for Pracas the paperduino:
I can't really tell what they did. Did they just print out the boards as templates, and then use wires to connect everything (inside the Cardboard)? I feel a "Carduino" coming on... 
You may get conductive ink if you mix it (worth a try) with the content of this
That's a bit pricey for only 7ml! :o