The bar for probable cause for CARB stuff isn't that high...
They can just say: exhaust smelled strong on idle. sounds loud/wrong. could see muffler was different.
When there's so many other things you can modify on your car, I don't understand why people are drawn towards the illegal ones. Unless you are timetrialing or in a race group or something, then just trailer your car in.
edit: it doesn't even fall under probable cause. the bar isn't even that high. you basically can't refuse.
http://www.expertlaw.com/forums/showthread.php?t=96096
https://www.dmv.ca.gov/portal/dmv/?1.../d2/c4/a1/2804