Wanting to know exactly how I "cheated" when i haven't gone online in several days

It takes time for the enforcement team to investigate reports. So the incident likely occurred weeks prior to the enforcement. You can file a case review if eligible. the enforcement team does not use the forums.

Happened to me. I was playing Mass Effect 3 and got a message from someone asking if I knew why his GS had been reset. I checked mine and it's gone except for 1 random ME3 achievement I gained today - and a few other people on my friends list have gamerscore
reset too. I haven't cheated - I don't even know how to cheat!

Codes for that offer haven't gone out yet and they haven't actually announced exactly when they will be sent out except that it will be closer to when Gears 4 is released.

I already have an Xbox Live, and have for some days. But I haven't gone online with it, just used it for normal play.

I don't think it's that severe in the eyes of the cheater. If you cared at all about your console or playing online, you wouldn't take the chance. They know the consequences. I think their desire to ruin other peoples fun overrides that. On a larger scale,
we could speculate for days on why that's the case. People want to be good at said activity. They can't be. They cheat. It's a disease of the mind that thankfully most of us don't have.