It could be a USB device(anything wireless?) or a USB pass-through to a monitor or something like that, or combination of both those things. If you have anything connected via a pass-through to monitor USB ports or anything like that, disconnect or move connected devices to a back port(on the PC) and see if it goes away. I've seen wake issues with wireless devices and pass-through USB connections causing PCs to wake despite all wake options being off, resolved by moving the device(s) to a port directly on the system.