Well, though it bothers me as well, hopefully there will be some sort of threshold defined so 1/100th of a gram of cola from 1987 doesn't have any bearing on the current situation. Also who knows if said past drug residue will even be detectable with this device..
Also, that cop will legally have to ask for permission to scan you or your car just as he does now to search you. It's just another form of searching.
As I'm sure you know, the best thing to do when getting pulled over besides being a polite ass kisser is to put your keys in your pocket when exiting the vehicle, after locking the doors..