Mark Twain is credited with the saying "It Ain’t What You Don’t Know That Gets You Into Trouble. It’s What You Know for Sure That Just Ain’t So." The more sure you are that your networks and information systems are locked down and comply with your security policies; that they are actually enforced by your nextgen firewalls, fancy URL filtering proxy servers, and SIEM dashboards, the more likely it is, that just ain't so. Shadow IT has a way of taking security policies that you know are Right, and making them Irrelevant because you can't even detect (or are not looking for) that shadow IT is bypassing your security policies and objectives.
A blog will accompany the book https://shadowit.weebly.com/ for information between updates to the book if you are interested.
Read the book for free: send me an email at djilpmh@protonmail.com and describe your suspicion of how Shadow IT is being used in your organization and I'll send you a coupon for the book. Example: Have you shut off the WiFi at home for "family time" or "quiet study/reading time"? So the kids are now using cellular data or the neighbors' WiFi instead, what do they care about your WiFi, and do you really want your kids to be so stupid and compliant they won't think of it?