Yep I saw the low 12v battery warning the first week after I got my FFE.
I then used my premium Sears 12v battery charger in my garage and it showed the level was down under 10%, so I charged it up to full.
I then bought a low cost 12v digital voltmeter that plugs into a 12v accessory port so I can monitor the 12v battery level daily.
For the past month its stayed about 12.4V even if the car hasn't been used for several days.
When it was too low it was well below 12.0V that one time.
I figure the 12v battery might have been low after sitting on the dealer lot for months and they should have done regular full charges like I did.
I plan to do a full 12v battery charge with my Sears charger once a month to keep the 12v battery in good condition.
Also since I can see the voltage easily now inside the car I will catch it early before it would be unable to go into ready to drive mode due to too low a voltage. I also keep a small 12v battery jumper kit in the back for emergency use, it has its own 12v battery that I charge up once a month with its own 110v power cord.
The 12V battery will charge while the HV battery is charging, but not after the HV battery is fully charged even while plugged in. It will charge a little in accessory mode (I measure voltage ~13.5V) and charge fully in ready to drive mode (~14.5V) but those modes will time out after awhile so you can't get a longer time 12v charge unless you are driving for a 30+ minutes. Hence why I plan to use my external Sears 12v charger once a month since I only drive the car for a longer (~25mile) trip once a week.
I say the reliance on the small 12v battery is the weak link in electric car designs today. They should give higher priority to maintaining this critical element by monitoring and charging it up as needed to maintain it even when in OFF mode, since there is still apparently some load on the 12v battery at all times for the wireless communications with the car that allow remote start and info updates.
Here is the little 12V LED voltmeter accessory I got from amazon, its a handy little device for the FFE:
http://www.amazon.com/gp/product/B0092K ... UTF8&psc=1