@tickub Yeah, I thought the exact same thing. It seems like he just figured it out, knowing the place thoroughly and knowing the girls as well (so that he won't be defeated in a hundred battles). An educated guess, as they say. However, he had a good chance to mess with their minds, so he explained it with the smell. That's how guys often behave, anyway.
I feel that Uchimaki is slowly getting there.. like I feel like this is development for sure xD Like he outright teased her before actually taking the chance to hold her hand to prove that he smelled the paint which represented her :) And since he's even taking more time to sniff it, he's surely taking more interest in Usami Man this is seriously such a cute moment ^^ Glad that Usami stayed in that cute position~ (Seriously each chapter Usami gets cuter and cuter!) Until Prez comes in to ruin it xD
Also Uchimaki didn't even let go of her hand when Prez noticed it xD He's either intentionally or unconsciously doing it and either way Makes shipper hearts really feel that pulse~ <3