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Paging through the site over the past days my impression has been that there isn't a denominator, it's just the total number of fevers reported. You might expect for this to increase some with the increased hypochondria, but as a parent of a toddler (who has a fancy forehead thermometer someone gave us), fevers aren't that hard to spot without one.
Anyway I don't know that that's the answer because as far as I can tell they don't actually say their methodology anywhere, but the maps I linked don't seem to be "per reading" or "per thermometer" or anything else. Just "fevers", "7-day trend in fevers", and "anomalous fevers".