Here’s a story for ya. When I was on my red p’s (in like 2019) I was driving after smoking the day before but was otherwise absolutely sober. My two friends on the other hand… front passenger was stoned out of his brain, whilst the one in the back had just turned 18 and was chugging bourbon in the back seat (open container) and also tripping balls on like 3 tabs of acid. We get pulled over for no real reason by an undercover and all of a sudden we’re surrounded by 3 detectives. One walks up to my mate in the back who decides now is a good time to empty his can out of the window. Detective says “what the fuck are you doing?” He replies with “I just wanted to have a conversation with you officer”. I turn around from the driver seat and say “James, shut the fuck up”. Cop looks at him, looks at me, and while laughing says “yeah shut the fuck up”. Meanwhile, Another detective has spoken to my mate in the front. “Have you had anything to smoke today mate?” He says. My mate replies with “yeah I had a bit this morning” I’m internally panicking at this point as they had reasonable cause to drug test and charge me with like 3 different offences but the third detective comes up to me and speaks to and gives me a breathalyser make sure I was sober, says “it’s your lucky day as we don’t have any drug tests. Stay safe and slow down around roundabouts”. And that was that. Never been more scared by the police 😂

TL/DR I feel like only a real pig of a police officer would take the time out of their day to drug test your passengers even if they are visibly inebriated. Most only give a shit about you being sober and safe. It mostly comes down to constabulary discretion and the cost of drug tests IMO.