FallenPhoenix said:damn man just don't open the hood. Unless the guy has reasonable suspicion that you've got drugs hidden under there or something else illegal, he doesn't have the right to search your car at all. Now for inspection you'll have to do something but don't worry about the cops that is unless he decides to just arrest you and impound the car...
Where I come from we dont say no to cops. If you say no dont open my hood, he's gonna call his supervisor, etc. and get the damn warrant if he has to.
IMO Unless a cop KNOWS Mustangs (in which case he'd probably let you off anyway) he's not gonna notice an aftermarket x pipe with cats, an aftermarket plenum, etc. He will notice the open element air filter, or that the car has no cats. I'm trying to cover the bases on the crap I know they look for.
Why is one approved and the other isnt? Simple. Getting your parts CARB approved is expensive, and some manufacturers just arent willing to pay to get it done.