Thanks for the hints.
I have found /var/lib/bluetooth/cache and removed its contents. Will test again next time I am in that car.
I ave also found that one can turn off caching of non-paired devices in /etc/bluetooth/main.conf.
Caveat: The only acceptable change to the caching is setting Cache=yes. Setting Cache=no makes bluetooth not start at all. The default is unset which sets Cache=always.