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.

MobileDeviceSU.pkgpackage for this update doesn't include any details. I'll see if I can get in front of a Mac that hasn't been updated yet so I can take a screenshot and add it to my answer. – Monomeeth May 29 '19 at 21:38