Keto Diet

In theory, those with a healthy metabolism who don't have excessive insulin coursing through their veins 24/7 can easily and naturally switch in & out of some level of ketosis. It's said that metabolic gold medalists, even those eating a carb-based diet, may have some traces of blood ketones in the morning, for example. The majority probably don't qualify these days, but that's possibly another topic.

Trace ketones = burning some fat and some glucose. Higher ketones = burning more fat and less glucose. The higher the ketones the 'deeper' you are in ketosis. Or the more fat is being used as a proportion of total fuel being burned. By that rationale I agree with @bulkbiker in that if you are measuring ketones, at all, then you are "in" ketosis. Or put another way, your circulating blood insulin is low enough to enable the mobilisation of some stored fat and/or the use of dietary fat. And after all, flatline insulin is actually the end goal!

My take is that if you're "fat adapted" rather than just "in ketosis" your body is acting as it should and will burn both carby
and fatty fuel as required and available, and the ketone levels you see will depend on proportions of carbs and fats you've eaten and what you've actually been burning for fuel - any carbs first then dietary fats, then, if required, body/stored fats. So ketone levels will always be fluctuating.