Once the vehicle has been connected to power and reaches the set charging goal, charging stops. The vehicle will slowly lose charge to power the onboard electronics - just like it would if it was parked without the power connection. And when it has dropped enough, the charger kicks in and brings it back to the desired charging level.
This is likely done to prevent the batteries staying at the same charging level.
Note that when parking for extended periods, the software appears to recognize extended time parking and reduces energy consumption to conserve battery charge.
We recently left our S 100D parked for two weeks disconnected from power - and still had a lot of charge left in the battery. Though you can also help reduce power consumption while parked by adjusting some of the settings (like disabling sentry mode).