My father was one of these. He lived across the country from us adult children and just seemed to be slowing down but an unrelated traumatic event exposed his early onset dementia acutely when his reactions to it were so strange. It also seemed to knock the facade off his familiarity with day to day life that was somewhat hiding his dementia. We then took him to the doctor and he was diagnosed.

Luckily he did not put up a fight when we took his keys but up to that point he was driving a giant truck around every day, seemingly mostly on instinct. I am still thankful that did not end in tragedy before we were able to move him into a controlled environment.