Yes, it comes from elsewhere in your body. A continuous glucose monitor checks the level of glucose in your blood. Glucose can be stored in your body as glycog

Yes, it comes from elsewhere in your body. A continuous glucose monitor checks the level of glucose in your blood. Glucose can be stored in your body as glycogen (e.g. in the liver, or muscle tissue). Your pancreas produces glucagon, which influences the process of converting it back into glucose, released into your blood. That's generally accepted basic science, but from my own anecdotal friends/family experience, the pancreas also seems like a pretty buggy organ generally, prone to glitches both temporary (like perhaps in this coffee drinker) and chronic (type 1 diabetes), leading to undefined behavior, crashes, and even fatally bricking the system permanently. So this anecdote isn't particularly surprising to me. (Also, for those reasons, I would definitely wear a continuous glucose monitor myself if it wasn't so expensive and difficult to arrange when you don't actually have anything known to be wrong with your blood sugar levels. I predict that within a decade or two, they will be a fairly ubiquitous piece of human body instrumentation, like the sleep-tracking wristwatches and rings we have today.)