Does the RADAR detector notify you of a certain band of frequencies (Such as K, Ka, POP, EVL, and so forth.), or does it just beep and go insane?
RADAR and the similar LIDAR and other technologies used by emergency vehicles today use radio waves at certain frequencies to either send a signal or bounce off of an object and return signal (the latter being how RADAR works to track your speed). Could it be that the Malibu's horn system emits a radio wave similar in frequency to those used by emergency vehicles?
Or, better yet, could the Malibu's horn system jam an incoming frequency?
Alright, here's an experiment (If you are ages 16-64, I would not recommend this):
Drive by your local highway patrol office when a cruiser is sitting outside. Call 911 and alert them of a reckless driver speeding in a [insert your vehicle's color] Honda Accord down the very street the office is.
When you come within view of the office, blow your horn and keep it going until you can no longer see the office in your rearview mirror.
Slow down to the speed limit and await your results.
If you see blue lights chasing an old couple in an Accord (which just so happen to be driving to the Honda dealership for warranty issues) then you can assume either A) You just tricked a government law enforcement officer or B) The horn system is a RADAR jammer.
If you see blue lights in your rear view mirror and they are actually after you, then either A) They are trying to alert you that your Audi has a broken horn and needs to be serviced or B) You're SOL.
Wow...that was a tangent.