| 
                         From the information you have given  and my understanding of it ( I may be wrong) 
I am thinking that the police car was following you ( eg moving at about the same speed) if correct the device must have been used whilst the police vehicle was moving and by a passenger. 
I have no idea what the local guidelines are for use of this device in the area where you were stopped, i suggest you do your own research and look for potential "breaches of the operating guidelines". 
eg  study the manufacturers operating instructions, imagine how these might apply to your circumstances. 
www.northants.police.uk/sites/default/files/Operat...f 
eg is a moving detection permitted ? or ONLY at a roadside check ? 
eg do the airwave radio systems HAVE to be turned off during the use of this device ? ( to prevent false readings etc)  remember there maybe more than one radio system in play a vehicle system and one for each officer. If they should have been turned off there should be proof of this somewhere in the case file eg a sentence in the evidence statement that says  " The device was used in accordance with the manufacturers instructions" 
                            
                                Edited by KenC on 05/05/2018 at 10:45
                             
                            
                     |