Is it not the case that when APTs "happen", they stay happening whether you detect them or not. That's the persistent threat part. The advanced part means you c

Is it not the case that when APTs "happen", they stay happening whether you detect them or not. That's the persistent threat part. The advanced part means you can't realistically do anything about it. The notion of unscrupulous or frivolous APTs (not just routine espionage) is quite frightening for that reason.