There have been studies on prairie dogs (or a similar small mammal) where they observed that, although to the human ear it was the same sound, there were subtle variations in pitch (or something like that) which the prairie dogs could distinguish between for different predators and react appropriately. For example, depending on the bird of prey, the prairie dogs either stayed still or dashed for cover; they knew what survival technique would work against which threat and not just "danger from above". Perhaps it's the same for guinea pigs?
It's interesting you say yours dived for cover. When I switched the lights on, my pigs were both hidden under their wooden log hidey!