I have "wake from usb" enabled in the bios. I'm using a wireless k/b+mouse combo.
On windows;
I can wake the PC from sleep mode by moving the mouse or pressing a key on the keyboard but if I power it off only the power button will turn it back on.
With Zorin, when I power it off, moving the mouse (to turn it over and switch it off) wakes the pc back up.
Do I have to hold the power button to "really" power off the PC?
Note: PC Specs in profile
How exactly do You turn Your Machine off? With the Button in the Startmenu? In the Quick Settings? Or directly with Your Power Button on Your Machine?
In power settings > power button > it is set to 'power off'.
If I use the mouse to power off or quickly press and release the physical power button I get the same popup with "power off, cancel, system will automatically power off in 60 seconds".
This does not put the PC in an S5 state. If it did it would not wake from bios events.
So for now I hold the physical power button until the system shuts down which is not the best way to be turning off a PC 
So when you have successfully "Shut Down" the pc and you move the mouse to turn it back on, does it go through the traditional boot up process (Bios, Zorin logo, etc?) or does it just go straight back to a login screen, AS IF it had been sleeping?
I'm only suggesting this while more experienced users help you troubleshoot, but to avoid the risks from a forced power off, you can use (in the terminal): sudo shutdown. Save everything first; if you don't add a time argument (sudo shutdown 5 to shut down in 5 minutes, for example), it shuts down pretty quickly.
If this ALSO wakes when your mouse moves... well, that's probably good troubleshooting information at least.
I have some kind of Feeling ...
Okay, could you test something. You already wrote that You have ''wake from USB'' enabled in your BIOS. Could You disable it and look if this will work? I know, this isn't what You actually want. It is only to see if this is the Reason or if there is something else.
This recent thread also may relate.
The O.P. suggests that these behaviors are different on Windows OS.
In the bios I changed "Wake up Event by" to OS instead of Bios. Now the mouse and k/b do not wake the pc when I "power off".
I'm currently waiting 15 minutes for it to automatically suspend to see if the mouse/kb will still wake it from that state.
Edit: And now it won't wake from suspend with the mouse keyboard.
It does wake, quickly from the suspend state, when I push the physical power button.
So,
If I use the OS to Shut Down, that is click the power icon on the screen which I set to shut down, the PC will wake from USB activity. I also noticed the PSU is not off. The fan is still running and the light inside it is still on. (Why the PSU has a light inside is beyond me. It's an old 1000W BFG)
If I use the terminal and the "sudo shutdown" command it will not wake from USB and the PSU is off.