It says in the article I read, Aiba accidentally locked himself out after unconsciously walking out of his hotel room. He says he was still half asleep when he went to use the bathroom and he didn't know that he just walked out of his room! With just his pants (underwear?) on, he couldn't go down to the lobby so he just slept in the hallway. It was Matsujun who woke him up the next morning. (Not related to the news about Mizukawa. Just another Aiba mishap. ^_^ It's a cute story though. It's so... him!)
Regarding the pictures of him with Mizukawa Asami, he clearly states "We are not in a relationship. She is just a friend."
Aiba is going to do a stage play entitled "Kimi to Miru Sen no Yume" which starts on May 2 and ends on May 24 at The Tokyo Globe.