More likely than not they will just make a notation in your file for use later and not tell you they saw it. It will likely only come up if you come in for some expensive warranty work, that's when they tell you they are not honoring the warranty because of the noted presence of a tuning device.
New Car: 2008 335xi Coupe
Earlier Car: 2004 STi (got stolen)
Earlier Earlier Car: 1995 Mustang GT (416 whp and a blown engine)
Really Much Earlier Car: 1988 Mazda 626 TURBO