He could have had a minor stroke that deprived his brain of enough oxygen to make him lose the plot and think he was driving the Duchess.
A dear old Auntie, in her early 80's, a few years ago, drove herself to church and then took a friend home from the church.
Despite driving O.K. she got lost and had to pull up and ask her friend (around the same age, maybe a little younger), where she was.
The situation was, there's no way she should have got lost, she was in very familiar territory, not far from her home. Her friend rang SWMBO right after she got home and told her she suspected Auntie had "had a turn" whilst driving.
Auntie told us the same story and couldn't figure out why she got lost and needed assistance and guidance. We took her to the doc and doc confirmed she'd quite likely had a mini-stroke that temporarily affected her awareness and cognitive reasoning.
She was quite O.K. for many months afterwards, but then had another minor stroke that seemed to knock her about and start her on the road to frailty.
She had a couple more minor strokes, and just as we were about to move her into a nursing home, she had a heart attack, and that was it.
The interesting thing was how she told us of various fantasy scenarios she had experienced whilst in bed, when she had the mini-strokes - such as giant insects under her bed.
We put it down to nightmares - looking back, it was probably hallucinations brought about by her oxygen-starved brain.