Laptop fried from spark while connected via USB to a battery system
Was it simply static build up on the vehicle?


Highly unlikely, because Laptop USB ports have protection chips against that.

But they usually connect the USB shielding directly to GND with close to zero resistance.

The following scenario seems a lot more likely:


USB cable screen only connected to +24V (or  +12V). That would put your metal case at the same voltage, which the 500k resistor would not mind much.
Connecing the laptop bridges the USB shield to USB ground line, which is at zero volts (due to being connected to your car electronics). 
Now there is a dead short of +12 or +24 volts inside your laptop, burning traces and probably leaving a quite noticable skid mark.


You could have prevented this (or at least noticed a burned fuse) by connecing the metal box directly to vehicle GND. There are isolated USB interfaces, but most of the cheaper ones only support full speed.