I left the bluetooth ODB-II dongle plugged into my 2013 SL for nearly 4 years without any issues with the 12V battery. I did not have an unit that turned off with the car or anything. I'll note that the SL comes with the little solar panel trickle charger for the 12V; although almost always my car was parked in a garage (at work and home). The only time I unplugged it was the two periods where the car sat unused for 2 weeks (I was out of the country), and I only unplugged it then because of concerns I got from this forum.
If you do decide to unplug the ODB-II dongle, do yourself a favor and get an ODB-II extension cord. Leave that plugged in and just unplug the dongle from that. This way you don't wear out the ODB-II port. I bought a Y extension, thinking that would let service or my state inspection plug into the free connection, but both always just unplugged the Y.
LEAF '13 SL+Prem (mfg 12/13, leased 4/14, bought 5/17, sold 11/18) 34K mi, AHr 58, SOH 87%
Tesla S 75D (3/17)
Tesla X 100D (12/18)
100K 100% BEV miles since '14
ICE free since '18