You can't manage what you haven't found.
The first job on every engagement we run is finding the assets the CMDB has lost. There are always more than the client expects.
The unknown estate isn't malice. It's history. A subsidiary the parent acquired four years ago. A lab network nobody decommissioned. Personal devices that wandered onto the wireless. Test environments that grew up; build pipelines that spun up VMs and never deleted them.
None of these things are visible to a vulnerability scan if the scan doesn't know they exist. So we discover first, catalogue second, automate third. In that order, every time.
"What was the last delta between our network discovery and our CMDB? Have we ever actually run the comparison?"