Unplugging is best practice to avoid the risk of power surges or other electrical faults from the wall socket to the charger. The chargers are designed to switch to a 'trickle feed' once the battery is detected as being fully charged. This simply means that the battery is monitored by the charger and topped up if a reduction of charge is detected. There is no problem with the charger being plugged in for long periods of time and no damage will occur to the battery either.
This feature is specific to the 1 hour dual chemistry chargers used by ONE+ and as the customer has specified our BCL14181H model this advice is relevant.
Thanks everyone for the replies as it made me push a bit harder for an official response from Ryobi.