I've no suggestions as to what happened, but some years back, I had a weird experience too. I woke to go to work, walked out to my car, and it wasn't there. "oh dear, it's been stolen" I said (well, words to that effect!). Anyway, I walked to work, reported it to the police, and asked the guys at work (Postmen) to keep an eye out in case it had been abandoned anywhere.
Later that morning, I was doing my own walk, and I saw it myself about 100 yards down the road - I don't go to work that way and it was on a curve, so I didn't notice it before.
I had the keys, there was no sign of a break in, and it was perfectly parked. People said I must have been drunk and parked it there by mistake, but (1) I didn't go out and (2) I don't drink and drive at all, so how did it get there? My own theory is that a bunch of rugby players manhandled it there, but to this day, I still can't explain it.