Every company that has a intrusion detection system will likely come across a false positive and/or false negative. If you've ever did test cases for your company, and the results gave you a false positive or false negative, could be because of a bug in the software, a failure in the hardware, or perhaps the functionality is not working properly. In any case, the hiring manager will ask this question to gauge if this problem is caused by manual intervention, or a failure of the systems. It's important to highlight your knowledge and understanding of this topic by talking about the steps you take to monitor the reliability of the hardware and software and use corrective actions to prevent future alerts of this kind. For example, did you have to do further testing or change the code or functionality in the software to correct this problem?