I can't currently find the place on Wikipedia where I learned this, but an example of "volume of chatter" being important was e.g. before the attack on pearl ha

I can't currently find the place on Wikipedia where I learned this, but an example of "volume of chatter" being important was e.g. before the attack on pearl harbor. While the US couldn't decrypt the internal messages, it was clear that an attack was imminent based on the pattern (including the radio silence) before the attack.