It's similar to the phenomenon of swearing you left your keys where you always do, and yet mysteriously they turn up in a totally different location, with no memory of how they got there and no possibility someone else moved them.
Most of our daily routines run on "automatic", meaning little or no processing of new information and operating strictly on stored representations in long term memory. It's part of how we are able to multitask, provided the task(s) is (are) well practiced.
I'm guessing OP was either greatly distracted thinking about the upcoming travel plans or was suddenly interrupted when he would have switched the car off and... didn't.
Its really easy to interfere with those automatic processes, (e.g., something happens that demands significant cognitive resources that interrupts a current task, and also disrupts the physical response behaviors involved in the task) and result in making huge, seemingly obvious errors without even realizing it.
So, don't sweat it. It happens.
Unless of course you find yourself doing that repeatedly, every single day, for many tasks... then you might want to see a neurologist
