As you may or may not know, cold temperatures alone do not harm the battery. However since the cold slows the chemical reactions, a cold soaked battery will be weak until it warms up. Battery weakness seems to be exponentially greater the further below freezing the battery temperature is. The FFE's battery must be big enough to minimize this as I have never noticed this even on a very cold soaked (sub 20F) battery. Granted I also have never jumped in a cold soaked FFE and floored it testing 1/4 mi times! What does harm a cold soaked (sub 32F) battery is charging it!
The cold alters the chemical reactions when charging such that some of the lithium ions plate out as lithium metal on the anode. This process is irreversible and as you can imagine with fewer lithium ions left, the capacity is permanently reduced.
The FFE's TMS kicks in heat when the battery temp is below 50F. Potential damage charging a cold soaked battery doesn't happen until below freezing. I do not know the exact details of how the FFE handles charging a cold battery. As far as I know the TMS will heat the battery to prevent or at least minimize any damage from cold charging. I don't know how realistic it is for the TMS to be able to heat the battery enough to avoid all damage from cold soaked to charging so I try to warm the battery thru use before charging. I'm probably just being overly paranoid here, but I also plan to have my FFE 10-20 years.
On the FFE, 100% charged is actually only about 90% SOC. So leaving it fully charged isn't as bad as it could be for the battery since it's technically only 90% charged.
Since charging a cold battery can be really damaging, more so than letting it sit at 90% SOC, I usually leave mine plugged in. I make sure to charge my FFE in the winter after a drive where the battery will be warmed up. Occasionally if I won't be driving my FFE for several days and I know I will have a long enough drive to warm the battery up before charging the next time I drive it, I will leave my FFE unplugged since I know the cold won't harm the battery. On second thought, that's probably not so smart since regen is charging and I don't think the FFE disables or reduces regen when the battery is cold. Someone correct me here if it does.
So yea, I guess the best answer is to keep it plugged in to keep the battery temperature above freezing. IIRC the reminder starts popping up when it's below freezing out. The important thing is to avoid charging and regen braking on a cold soaked battery.
If you need to park unplugged for days just try to avoid using regen and be sure to get the battery warmed up some before plugging it in to charge. I may be curious enough to check my battery temperatures periodically this winter and report back here. Others have reported that the battery tends to cool slowly so it may still be warmer than you would think after sitting in the parking lot all day at work.