Spoke too soon. The problem is back. At the moment, my best guess is that it's Skype that's causing it, because it was only after I launched Skype for the first time since the reboot that the problem came back, but who knows.
When I killed the battery and then booted in Safe Mode, it was definitely gone though.
So I guess now it's just a matter of figuring out which app that launches at startup is causing this