Definitely something with filter capacitors, likely something(s) electronic: VHF, AM/FM radio
Way back I worked on on a friend's boat that had a "battery eliminator" 12V supply intended for motor
homes or trailers - it was basically a transformer-rectifier that put out about 13V RMS... when not filtered. Unfortunately connecting it in his boat added his radios' filter capacitors to the circuit, bringing the DC voltage up to nearly the 1.4 PEAK - something around 17-18 volts, which eventually fried his batteries (not for the first time).
He was looking for a cheap
solution, but a regulator
would have cost more than replacing the "battery eliminator" with a proper charger
In your case, it's not a problem, just the way things work. You can figure out who/what is contributing the capacitance by switching off or disconnecting things one by one and repeating the battery-switch test.