Maybe the accused were going to be sent for a further blood test to more accurately determine drug levels or something like that and they thought better to come clean now than look like a dick when they're caught out for further substance abuse, whether it would've been detected or not.
Maybe they were caught on video. Nitrous has a half-life of like 5 minutes. They say it can't be tested in "routine tests" but I'm pretty sure it just can't be tested.
I don't care if they use weed, but they are enforcing the UK's shitty weed laws. It's shitty to then turn around and smoke yourself, after you bust someone for a few grams
"Met" as an abbreviation always throws me off. I thought this post was about some dudes that OP met, and I wondered how you end up meeting people during their drug tests.