Here's a pic of the sensors used in an Audi's rear parking sensor system.
They are the small round things in the bumper cover.
Why can't the aftermarket "parking sensor" makers (aka laser jammers) use heads like this? They're quite stealth, and if you're caught, you can really say they're parking sensors.
Even if they have to be dark colored to pass IR light, why not make jammer heads that are tiny like that?