I don’t think its rpi or network switch, unless you’ve overclocked rpi with liquid nitrogen 😅. So, I assume its TrueNas device.

If it were a significant power difference, say 20-30 watts, you could easily find the process using htop/iotop. However, 6 watt difference is a relatively small value for a device with ~25 watts of idle power . It might be a process using just 1% system resources. That’s why I would look for systemd timers, cronjobs etc. to find scheduled tasks on specific times. Another possibility is automated S.M.A.R.T. self-tests. Those tests don’t show up in htop or iotop.