Interstellar Factors from the Contacts in station services in a low security system.
I'm not sure that small fines become bounties, but if they do, they will have a timer of 8 minutes before they become a legacy fine, so it's highly unlikely that a player would ever notice that they became a bounty. They would not only have to be playing at the moment the fine became a bounty, but they would also have to be in the system where the bounty was active to notice that they became wanted, and once the 8 minute timer expires, it's gone.