The initial releases of macOS Sierra and High Sierra Security Updates 2019-002 on Monday 25 March were not complete. Apple has now released updates to both of these: if you applied either security update before 29 March, you should install this revised update when you can.
Apple states that current updated build numbers you should see are:
- for Sierra, 16G1918;
- for High Sierra, 17G6030.
If your system shows an earlier build number, you need to install the updated security update.
Those who updated Mojave to 10.14.4 aren’t affected by this.
(Thanks to Al for alerting me to this.)