To be clear I have been having this issue also. And I can even be 100% sure:
this happened to me directly after a fresh reset, my usage was at 100% hourly and weekly, I was NOT using codex, but I had it open because I was planning to. I was reading the current diff to remember what I was working on previously, and then when I got to codex, my hourly and weekly usage drained – hourly down to 80%.
I, still having not used codex in a few days(no messages at all), deciding to start writing a post. ( What automatically consumes Codex usage without doing anything? - #5 by EricGT )
While I was writing that post, I observed the usage further going down, despite no usage being done yet.