It could be that it may have happened during the night and missed it. You would have to had to mix 2 different anemones. 1 female and 1 male. You never know what can happen in your tank. Or maybe a small piece of the foot with no tentacles somehow broke off then started to grow new tentacles making it appear as a baby anemone. It could have gone either way, the tentacles look pretty small from the picture.