When people are passing in a car we are only treated to a snapshot impression of them - so our minds tend to fill in the rest. (So I would imagine anyway). So the postures and attitudes the passengers happen to be assuming tend to take on overemphasised significance.
With this in mind, I'd go with the bicycle thieves scenario - and their rigid posture (only seen at a glimpse anyway) explained by their desire not to give anything away.
The question: `Have you seen anything unusual?` is a standard cop one (at least in TV shows it is), and not one intended to elicit mysterious stories!
The two witnesses were buzzed by an early ( and I would assume infrequent) dose of yeast derived products. And villains tend to drive loud old bangers (from TV again!). So I don't find anythin g too inexplicable in this superb tale.
I have my own which is vaguely similar. When I was about 11/12 I used to go on early evening bike rides on my own along the roads criss crossing the fields adjoining Southport and leading to Ormskirk.
On one of these evenings -it was early dusk - a man drove past me in a Reliant Robin or some such basic, cheapo type of car. As he passed I noticed - or seemed to notice -that his head was lolling back on the seat behind him - as though he were severely disabled and had a broken neck - and he was looking straight upwards! He also wore dark glasses of some kind.
In my overheated child's mind the man was disabled and the glasses he was wearing featured some type of periscope type arrangement that allowed him to look ahead - despite his head being stuck in a facing-up position - thus allowing him to drive. This struck a chill into my heart as it confirmed that the World is a place of Scary Weirdness (which is what I wanted!)
This attitude now seems like a massive non-sequitir: if such disabled people were enabled to drive by such a contraption...wouldn't that be lovely!
It now seems, in my banality soaked adullthood, however, that in all likelihood I had just seen A.N Ordinary geezer who just happened to be temporarily looking upwards as he passed me.