The novelization of Alien has an interesting discussion between Ash and... I think it was Dallas but it could have been Ripley, about this topic. Anyways, Dallas goes and confronts Ash about keeping the alien alive because the autodoc should have picked up on the embryo, especially when like you said it is about 8'' long. Ash says that the alien feeds off of nutruients, glucose, adipose tissue and some other stuff, but that the autodoc did not detect it because the total mass was kept in equilibrium. Now aside from Ash obviously covering it up, does that help at all?