One of my computers is a MacBook Pro, mid 2009 that is running 10.11.6, El Capitan (the latest that it will run). Last update was a Safari v 11.0.2 installed 9 Jan. I have a message that Security Update 2017-005 is available and recommended.
When I click the update button, Restarting Your Computer shows. After 1 minute, or if I click restart sooner, a box pops up with “Available updates have changed. Click “Show Details” to see available updates”. When I click “show Details” I am back where I started with the 2017-005 available message.