Read Alex Moyes explain what company boards should demand of cyber security on Infosec Magazine :
If there is an active intruder in your network, you would likely not know. Like most organizations, you would be in the dark. It is entirely possible that months or even years ago, an outsider gained access to the network and quietly began to explore it and expand their area of control. Over the first weeks and months, the attacker gained access to all the servers and began looking for files that might have value.
Read his full article here.