Shukan Josei (which will probably sponsor their marriage when the time comes) reports that Matsumoto Jun attended the opening day of 'Miwa', a stage play which Inoue Mao is in. The play opened on 4th October at the Tokyo Metropolitan Theatre.
He was wearing a black hat, glasses and a cardigan, seated in the section reserved for officials and staff members. Next to him was actor Kasahara Hideyuki.
A little about Kasahara Hideyuki. Kasahara is Oguri Shun's junior in talent agency Tristone Entertainment. Matsujun and Kasahara grew closer after co-starring in 2010 drama 'Natsu no Koi wa Nijiiro ni Kagayaku'. Before that, they had actually at times met at Oguri's home but Matsujun had no idea that he was an actor until they were cast together. Kasahara has been close friends with Mao since she was in junior high. They have co-starred in dramas Juui Dolittle & Tokkan and the movie Shazai no Ousama. It seems he often gets teased by Mao during filming. Looking up recent entries in his blog, a staff member posted a photo of him and Mao on 7th October but it's probably a shot from the time of Shazai no Ousama as there's no mention of 'Miwa' at all.
Shukan Josei went on to say that after the performance, Matsujun went backstage to meet with the actors including Eita, who he co-starred with in 'Lucky Seven'. On seeing Mao, Matsujun said "Here are the cakes," and handed over a paper bag with enough for the 40 or so staff members and actors. Mao said,"Yeah, thanks" and proceeded to give them out. The article highlighted how familiar they are together, like they are a married couple already. And that's about it.