I have a bit of a mystery. Every now and then, as I'm working on my iMac running High Sierra, I occasionally see a box pop up that says "Opening BaseSystem.dmg," then it disappears within a second. I never saw it until installing the latest macOS security update released this week. I don't think it's malware. It's a clean system. I don't visit sketchy or nefarious websites, nor do I download things I shouldn't. All my software except for Firefox, Brave Browser, and Capture One are from the official App Store, and the ones listed previously are all from legit developers. MalwareBytes scans come up clean. Anyone else run into this? Any ideas or solutions?