There's going to be a few different opinions on this. I didn't see anything in the official manual. I've always left the car connected to power, primarily so that it will pull from the mains if it needs to run battery conditioning (more so a concern of mine in the winter). Typical wisdom is to discharge to below 80% but above 30% and leave the vehicle unplugged. The longest mine's ever sat is about 2 weeks, however.
I have a 2013 smart ed and for the last last four years I have left my smartie plugged in for four six month periods of time. I plugged the hv battery into my Siemens 240v 40amp charger and the 12v battery to a trickle charger. I have had no problems after each of the six month periods.
How much charge did it lose in 2 weeks? Mine would be dead flat 0% in 10 days if I did that, and I am having a fight with Smart Car about that. Their position is 10 percentage points per day parasitic loss is not a lot and they won't do anything to fix it. Dealer says they can't find anything wrong.
My brief experience is that 10 days sitting at about 40F gave me about 7% SOC loss. I would not recommend leaving a Smart ED unplugged for six months because of the risk of self-discharge damaging the battery.
I think that our colleague Be A Star has an actual problem, like a malfunctioning battery heater or coolant controller, which causes her vehicle to lose charge quickly.
If I had to choose between the risk of damage from storing the vehicle hot and fully charged, or the risk of full discharge damage, I'd choose to leave it on a charger.
Better yet, rent or loan that car out to somebody who can use it !
I have left my electric motor scooters over the winter in a garage at about 60% SOC for up to 3 months or so. No evidence of discharge at all - and these are cheaper Chinese cells, not German ones or wherever Smart gets theirs.
Bottom line, if the car won't experience temperatures below -20C or above +35C, it is probably fine to leave the car sitting at at least 70% SOC for 6 months.