Apple - How can I confirm macOS received a silent update?

Your question did not specify a macOS version, but in macOS Mojave, background updates such as Gatekeeper or MRT (the Apple malware removal tool) will typically show in System Information.app -> Software -> Installations.

Source: About background updates in macOS Mojave

Do this in a Terminal window:

softwareupdate --history

If that does not work, do this:

grep Installed /var/log/install.log