/etc/acpi/events/asus-brightness-down is in acpi-support 0.140.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | event=hotkey (ATKD|HOTK) 0000002[0123456789abcdef]
action=/etc/acpi/asus-brn-down.sh
|