Well, a voice will call your SMS (cell phone) and tell you which area of the city they are in. This happens at random, usually when you pass from one area to another (A little message will appear on the screen saying which area you are entering, ex. "Beacon Hills West.")
This only happens if there are any hidden races remaining.
Once you get the message, you drive around that area of town and look CAREFULLY for a race that isn't on your mini-map. Sometimes they will be impossible to find.
You CAN find these races without being given a message, it just requires patience, time and effort. --- XBL GT: dark minun Claiming dead boards - a passtime for those who are scared to interact with others over the internet.