cars designed to drive 100s of thousands of miles aren't susceptible to random splashes even if the battery is low. I think the most likely explanation here is

cars designed to drive 100s of thousands of miles aren't susceptible to random splashes even if the battery is low. I think the most likely explanation here is that the battery compartment was damaged in a way that allows water intrusion, and then they drove in extremely wet weather which splashed into the compartment.