A relative died in Hancock County, Illinois, in the 12 month period before the enumeration of the 1870 US population census. She should have been listed in the mortality census that was taken for that year and I was anxious to find the reference to her.
Online indexes did not make it “obvious” that the mortality schedule from 1870 are not extant for Hancock County. After searching for a few minutes and not finding her, I decided to look at the National Archives website to see what counties in Illinois had extant mortality schedules for 1870. The alphabetical list of counties started with Kendall. That explained why I could not find the person for whom I was looking.
Before you spend too much time looking for that elusive person in an online index, make certain the records for the specific location you are looking for are actually included.