Apparently I got a Gold account for a while. The issue is that it seems to have ran exclusively during a period of time in which I wasn't even using the site.

I'm 100% fine with it staying gone, but would it be possible to get that back now that I'm actually online? I'm just bewildered - where'd it even come from if I wasn't even signed in when it was active? Was there some kind of event I missed?

Somebody probably sent you a buddy gold account. One of those nice things that subscribers can do for others. They run automatically when gifted, and the sidebar notice is easy to miss.

I always ask 1st before gifting a buddy GA.