You have batteries and not power supplies... Is anything else connected to the batteries which is joining the grounds?
If not then what is happening is that the two batteries are not at the same ground potential as their grounds are not connected. Both are making 12V potential difference between their terminals but one battery relative to the other battery is not at the same potential. Likely also at a different potential to you, to the tree nearby etc. So when you touch the camera enclosure with the USB connector you basically connect the grounds together and a small current can flow over the air to equalise the two ground potentials. During this the electrons hit the air molecules and this produces light (a spark). The spark won't happen every time as the potential difference depends on the humidity, battery handling etc. Too small potential difference = no spark. As a rule of thumb, spark over 3mm gap corresponds to 1000V potential difference.
You will probably also see a spark if you just take a wire and join the two negative terminals of the batteries. Again a small current will flow to equalise the two ground potentials...
(just be careful connecting battery terminals, accidentally shorting the +ve and -ve terminals WILL HAVE BAD CONSEQUENCES as the (lead acid?) batteries can produce LOTS of current).
Anyway, there is still no ground loop as there is only one ground path from one battery to the other one:
battery 1 ground -> camera power ground (camera PCB ground) -> USB connector ground -> laptop ground -> battery 2 ground.
My setup is (still) in my backyard, powered from power supplies. I have never seen sparks but the connectors I used are "hiding" the wiring so it won't be easy to see sparks.
But I would not be surprised to see them when connecting the switchmode power supply (floating potential) to the laptop USB (another floating potential, the laptop mains plug has no ground).
Hope this helps (sorry about the long explanation, just looking for distractions from work
)