I was able to narrow down the search area to one room. The signal never peaked to "strongest" until I held the Fitbit One next to the iPhone. I suggest that the graph scale be reduced to show a peaking signal better.
Clicked on "Found it" and the app demanded a tip for karmas sake. I may have given one if it was to a charity.
RVKnudson about Fitbit Finder - lost Fitbit, v1.5