I have known people with dementia, both senility and Alzheimer's. I've seen them forget something cooking on the stove; tell the same stories again and again within an hour's time; forget where they parked a car (but knowing they have forgotten and can't find it); not recognize close family members or think their daughter was their wife (close resemblance, give or take 25 years); flush false teeth down the toilet, etc. I have never seen or heard of someone undertaking a really complex action like taking the car to a service station for gas, then leaving it and walking home without it. That sounds more like a variation on sleepwalking. Whatever it is, I wish them luck. Watching someone you love, even a dog, with any kind of dementia is a terrible thing.