I don't think its rpi or network switch, unless you've overclocked rpi with liquid nitrogen ๐ . 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.
Biggest one: Aldnoah Zero
Some potential contenders are: The Detective Is Already Dead, Akame ga Kill, and Shield Hero.