Did the car still have gas in it? Do you supposed that she committed suicide?
If she did, then it’s a fair bet that SOMEBODY knew she was dead. ignition keys do not turn themselves off and slide part of the way out of the ignition slot.
Could be carbon monoxide poisoning. It can happen if the car is improperly maintained. Its known as the “silent killer.” But its odd no one noticed for six years and never bothered to check the garage.
People die all the time in this country especially with no one to look after them, without a second thought if they have no relatives. I’m not surprised one thought about the obvious all that time.