Does look like a keiki, but personally with it so close to the medium I would leave it to become part of the main plant. On a sympodial orchid like this a keiki is just a new growth coming out slightly higher than usual.
Thanks Rosie, I think you are right. I think this one's growth is different because it's actually another pbulb that's just started further up. I intend to leave it on the main plant as you suggest!