Every once in a rare while, for a split second, a dialog box will appear on my Mac saying that it's opening BaseSystem.dmg, and then the dialog automatically vanishes. The system itself is working fine, but there's no explanation of why this happened, what it's doing, or if it's a security risk.

Empirical evidence suggests they appear shortly after doing a system update or security patch, at other times it may appear after a Time Machine backup completes.

Turns out, this file does exist on Apple’s Time Machine volume(s). (Below /0/ may be /1/ on your system.)

/Volumes/YourTimeMachineVolumeName/Backups.backupdb/.RecoverySets/0/com.apple.recovery.boot/BaseSystem.dmg

The .RecoverySets directory contains boot information, EFI, and a lot of kernel material. It has that “if you need to reinstall on a blank system using just a TimeMachine volume alone, you can” feel to it.

You’ll also find it in places like:

/Applications/Install macOS Catalina.app/Contents/SharedSupport/BaseSystem.dmg

If you locate the disk image file, mount it, and look inside …it is indeed all the minimal stuff needed to reinstall the operating system.

Why macOS would need to open and mount this is unclear, perhaps it’s pulling or checking a resource from it. But it doesn’t appear to be a need for concern, other than it’s an annoyance if you’re trying to give a presentation or watch something full screen.