Moved In With the Children?

If late in their lives, you can't trace one of your older relatives, consider the possibility that Grandma moved in with one of her children or grandchildren. This could easily have been a distance from where she lived most of her life and where her husband is buried. It is also possible that a "disappearing" grandparent moved near one of their own siblings instead of one of their own chidlren.

It's also possible that your "disappearing" older relatives moved away and did not live near any of their children as a pair of mine did in the 1870s.