A Detection Deficit is the gap between the times it takes to ‘Discover’ a breach from the time of ‘Compromise’. The detection deficit distracts you from looking for the breach.
For example, A hacker hacks your computer. It takes time for you to figure out that you’ve been hacked. During that time the detection deficit is launched and distracts you from figuring and finding out that you have been hacked.
Later, when the hackers have all the information that they want from your computer they release the detection deficit. You go and look for the breach but it is too late. You information is stolen. That time is called Detection Deficit.
