Suppose that you have 3 data points, on June 14, 15, and 16, that due to personal driving quirks all appear to belong to the same person. If the 14th and 16th are in your dataset, and both correspond to Alzheimer-free Bob, that may be a strong hint that the data from the 15th is also Alzheimer-free.

But this doesn't help you in the real world where you won't necessarily have near neighbors corresponding to the same person, with a known diagnosis.