It was Isaac! He had managed to get all the way upstairs and was running around when I realised and went to get him. I had only turned my back long enough to get tea out of the oven and our kitchen and living room are attached so its not like he was unattended. He is just really quick! Gave me quite a fright and reminded me to remain vigilant at all times. No real point to this just a reminder i suppose to close the doors.