Years ago, when I was still new to research, I was working on a family where the husband purchased federal land in 1849–about 200 miles from where he and his wife (and her extended family) were living. They had been married about three years when the property purchase was made. The couple were easily located in the 1850 census and in later records in that county until his death.
A search of the 1850 census for the county did not turn up anyone listed with the wife’s last name. Based upon that, I assumed that her family did not move to the county with them.
The wife died before any “good” vital records were kept in the location where she died and extensive searches for her family members in the county where she and her husband married (three years before their move) located nothing.
Turned out that her family had followed them to the county where the land was purchased–approximately 1852. After the census date. That assumption that “her family didn’t move there” was an incorrect one made early in my research, simply because they were not in the 1850 census. They simply had not had time to move.