When I did my ADR course the explanation given for the scenario we're talking about (or any domestic probably) is no sign is required. If the FB or any emergency service turn up to a RTA/RTC and see a vehicle with a warning sign then different rules come into play because by rights you havn't got one dive cylinder in your car; you're carrying a load of cylinders. So, any injured people involved will now be waiting a little bit longer before the emergency services can intervine. Of course this was a long time so I won't swear it's not changed. I have a rather zelous official at work who did the course more recently and now thinks carrying 20lts of petrol in a jerry can requires the worlds amount of paperwork and signage. As has been pointed out already in this thread, there's probably four times more fuel in the car.

So unless somethings changed, it used to be about quantity carried rather than how it's carried.