Where else would a hidden idol be other than the incognito driving seat of any situation.  Yu probably needs those specs moving forward, but he can't see everything.  Thirst can do this to a person... Trust is everything, I'm sure it's all in playful fun.
 
A bonus item from PM, the Persona Magazine subscription publication from ASCII Media Works (Kadokawa). This colourful work is a Bromide-format Art card, slightly smaller than postcard size often given away for promotion or a purchase bonus.
 
~This item qualifies for Lettermail super affordable shipping~!