Cops are trained on attitude, etc but most wouldn't have a clue what to look for to find if somethings hidden. Some cops wouldn't even know what a boost controller soleniod looks like, but others will. It's the luck of the draw. The cop who defected my old 323 didn't have a clue (a boost gauge is not a rev tacho). Don't make your car stand out like a sore thumb.
If you modify your car in a smart way (eg. spraying your front mount black), then you can get away with basic defectable modifications, as in this thread, the guy got away with a top mount turbo because his engine bay didn't have a sea of chrome. But he did get done for other issues.