According to Bloomberg Businessweek contributor Paul Barrett, there are two types of corporations: "those that have been hacked and those that don't know yet they've been hacked." In the case of Sony Pictures, the former is the most applicable description.
But this has not always been the case. Sony definitely didn't know when it was being hacked or for how long it had been happening when the company's IT department was found to be severely lacking in function and security. According to Gizmodo contributor Ashley Feinberg, one document leaked in the Sony Pictures hack revealed that there was serious concern - and little action - regarding organizational IT shortcomings.