Exploring possibilities of being hacked

Hello Mac friends,

I am not an expert by ANY means, so I apologize in advance for how uneducated this will sound. I'm just a person with suspicions of being hacked looking for help...(by someone who I lived with and had access and personal details enough to do so). I am not looking for solutions to get rid of it (yet), my intention is to find it, pin point it, and catch it if it's there. 

1. Is it possible for my entire OS to be cloned/duplicated/mirrored/split (whatever the proper term would be) and be HIDDEN? If so, is it possible to find evidence of that somewhere on my system?

2. Is it possible for a device to be hidden that is logged into my iCloud/iTunes/Apple ID?

3. Is it possible for someone to be hiding within one an app, file, bluetooth, etc, on one of my apple devices - and in doing so, gain access to my iCloud/iTunes/andor Apple ID? 

I'm trying to determine what possibilities exist in the world of hacking and network security that could explain how iCloud data shows an IP address currently logged in associated with Verizon. I know this log in is not me. My devices are all accounted for. iCloud has no alerts on new devices etc. 

Does anything else exist out there that come to mind for you? I appreciate any suggestions or ideas. Thank you thank you.