You’re right, it isn’t detectable, but it is also a short-lived high, so if someone isn’t going to keep on inhaling it while they’re driving, they shouldn’t continue to be impaired, and should present less of a danger on the roads (I’m not excusing them BTW).
If they’re going to decant it into balloons and inhale it while continuing to drive, I’d expect plod to be able to find discarded ballons in the car which will probably have residual amounts of the gas in them, which could then be used to prosecute.