There's a post here: whatconsumer.co.uk whose top post seems to be a quote from a BBC page that no longer exists. It covers the basic issue that there was in the Grand Scenic II. Basically, there was an electric panel that hundreds of customers had issues with. All the electrics in the dash would turn off or flicker while the car was in motion, meaning that the drivers could not determine gas levels, check engine issues, or even their speed.
It's a problem that only seems to affect Scenic II models made in 2004 and 2005.
It appears that by the time the Scenic III came out (in 2009), these problems were totally resolved.
Most likely, your salesman was just trying to show off his knowledge. Typical salesman!