If Grandma or Grandpa "evaporates" after the death of their spouse, make certain you have searched for all their children, not just the one who is your direct line. Your ancestor could easily have moved in with one of their adult children after the death of their spouse.
Find all the children of your ancestor. Look for them in census records. In pre-1850 censuses, Grandma or Grandpa may appear as a "tickmark" in one of the older columns. Grandma or Grandma or Grandpa may also appear in the cemetery next to one of those children. If they moved several states away to live with a child, they might not have been taken "home" for burial.