I'm no expert on this, so I am just thinking out loud here.
All our bodies produce a moderate level of ketones, this is normal. Ketosis is the term used only when this level is elevated.
Ketones are produced by the liver when breaking down fatty acids. Therefore, I'm assuming that there could be certain things that will somehow block access to the liver's stored glycogen for some reason. This could give an elevated amount of ketones for a short period of time, but I'd imagine that it wouldn't be as high or as constant as being in ketosis on a VLCD.
Having just read more about ketosis, ketogenisis and ketones it's quite amazing how the body adjusts and adapts through it all.
It seems that it's the brain that makes us feel hungry because of its requirements of glucose in the blood. After 3 days it gets 30% of its energy from ketones. This gradually rises to 70% after 40 days. This is obviously why the beginning is so difficult and then it gets easier as the brain adapts.