What I know:
- Component of "Software Update" (and is visible when you open the list).
- Requires reboot
- 108 MB (for Sierra)
- is hidden in the "Updates Installed in Last 30 days list
- (probably) is not part of the auto-installing "Install system data and security updates" track
First appeared with "macOS Mojave 10.14.5, Security Update 2019-003 High Sierra, Security Update 2019-003 Sierra".
Tried Apple sites and some basic googling.