Before you park the car at the place where you picked it up you...
Precisely. This will ensure the next user doesn't have to refuel immediately or (in some Flemish cities) runs out of battery. If you must refuel during your trip, you fill it up entirely. This is helpful for the next user.
Alas! This really is your responsibility. If you ensure that the tank is at least a quarter full or (in some Flemish cities) that the electric car is recharging, then the next user can drive off immediately. If you must refuel, you might as well fill it up completely.
That's very thoughtful of you and of great help for the next user. But if the tank is more than a quarter full, you actually don’t need to refuel. An electric car (available in some Flemish cities) should only be reconnected to the charging station.