On Mon, Jan 29, 2018 at 10:04:39PM +0100, Martin Wagner wrote:

The following messages have started to appear in the system log after the most recent update of my Centos7 machine

gnome-software-service.desktop[3389]: 20:54:22:0879 Gs no app for changed places-menu@gnome-shell-extensions.gcampax.github.com gnome-software-service.desktop[3389]: 20:54:22:0886 Gs no app for changed alternate-tab@gnome-shell-extensions.gcampax.github.com gnome-software-service.desktop[3389]: 20:54:22:0898 Gs no app for changed launch-new-instance@gnome-shell-extensions.gcampax.github.com

Those are the first line of multi-line journal messages, sent by gnome-shell's javascript interpreter, most likely on the behalf of the mentioned extensions. There might be more information in the later part of the messages in your journal.

You can debug the extensions by starting GNOME Looking Glass, run it by typing Alt-F2, type 'lg' and hit return.