A master is out is an interesting meme which is actually two different memes depending on what side of the Pacific you're on.
The American "master is out"
I guess I have to explain this one first even though it only makes sense after you read the second section. When 4chan was first started, a Japanese user of Futaba's /b/ tried to introduce the immensely funny "a master is out" meme to 4chan's fledgling userbase. Unfortunately, he had no knowledge of English, and only a machine translator to attempt to get the message across. So, the phrase which could probably be best rendered either "Where did my master go?" or "The dog is roaming around" was transmogrified into "a master is out." This meme was probably only posted once by the Japanese guy; 2003 was a time when any nonsense that landed in /b/ became instant comedy which had to immediately be repeated across all boards. "A master is out" had a long shelf-life because the dog image that accompanied it was cute and silly, but the intended meaning was lost before it even got off the ground.
The Japanese "master is out"
I have a single, unconfirmed report that when this image is posted on Futaba's /b/, it is a signal for "Roll Call!" and immediately everyone posts a reply with images of famous anime characters claiming to be the dog's master. Now that I think about it, I'm not sure whether this is actually true. I do know that Futaba loves shiba inu (the breed of dog pictured here) deeply so this might be a sort of lolcats image mainly known for how frequently it's reposted. One thing's for sure, the phrase "a master is out" has actual semantic meaning on the Japanese side of the ocean.