Many thanks to @chanezmeguettef for sharing on twitter.
A fan has shared a series of photos that she took with Keanu as he was leaving the gym on August 26, 2013. She appears to have approached him in his car for an autograph. Being the nice guy he is, he got out and took a photo with her.
Photos are here, here, and here.
Update: @chanezmeguettef has tweeted a whole series of photos of Keanu from the day she ran into him at the gym. You can view them here, here, here, here, here, and here.