I'm currently abroad with a notification for "November Pixel Drop update available" that appeared the day following my arrival. I believe I had already installed the November update back home earlier in the month. Every time I go back home, a couple of days later I get an update too.

I'm not claiming to know of any foul play, but it has happened several times, enough for me to notice. If it was related to time of the month, it wouldn't be as consistent. It might be that you need specific combination of phone, configuration and network provider for this to happen. Maybe I've been p0wnd, but I've noticed this behavior since at least the Nexus line.