It happens, tragically, not all that uncommonly and is a well known psychological phenomenon. Basically it does involve having a heavily ingrained autopilot routine and then deviating from that routine (like being supposed to drop someone off) and then usually a distraction of some kind... People then just default to their usual routine.
It actually happens to me the other way around quite a lot. Youngest has only recently started doing two full days at nursery, she used to only do afternoons, so I'm used to her being around in the morning. A couple of times I've had a couple of seconds of panic when it's really quiet and I can't find her before I remember she's out. And once I was doing the garden and left the front door lying open (clearly knew she was at nursery when I actually left it open), went upstairs for a while, came backs down and saw the door lying open and totally crapped myself for a minute thinking she would be out on the road
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The brain works in strange ways. It's more based on routines and patterns than people realise... You spend more than half of your life not actually thinking about what you're doing at any one time.