I suggest to check for a setting in the BIOS that is named "Wake On LAN" and then disable it.
This feature is often (always?) enabled by default and allows the computer to start via the LAN connection. I had "Wake On LAN" enabled on my Z800 and started that computer by mistake with my laptop in another room. After disabling "Wake On LAN" in the BIOS of the Z800 that never happened again.
Worth looking for! 