Monitor turns on again when screen is locked
Using Debian sid/unstable with gnome-shell 42.3.1-2 (but happened also before), locking the screen the monitor goes black as expected. When returning after a while, often the screen is active and the time/clock is shown. This is unexpected, and I am certain nobody moved the mouse/touchpad. Also, the monitor does not go black again after a while.
How can I find out, what event caused the screen to wake up?