The slight nod of acknowledgement that "yes, I see you are also a foreigner" when meeting other foreigners in Japan. In fact, it happened more in China this summer. I find it quite funny as it happens to people you in other situations would have nothing in common with. Like in a bar in Nanjing, two suits in their 40s, leaving the venue and giving us the nod. Or in a club in Xi'an, a group of Turks having a nod and a chat. They included a guy of 2,10 meters and almost equally broad, so it felt prudent to be on the good side of them.
I guess when being in a country this homogenous, with Japan sporting 99% Japanese (and of the remaining 1 % the majority are Koreans and Chinese) making even Norway seem multicultural in comparion, it feels necessary for outsiders to have their own little club of togetherness, no matter how strange it is. So the nod, it's on.