I've been with Adsense since about 2005. This morning I received a strange message while logging in to check my account. It said there was "unusual activity" and that I needed to enter my phone number or the city which I typically login from. I got past everything and into my account. I changed my password just in case it was comprised. Any thoughts on this? What could it mean? Anything else I should look at? I'm a little scared about this.
Possibly someone tried to get into your Google account. If you use gmail, I strongly suggest you go with 2-step authentication; it's a royal PITA but it's better than getting your Google stuff hacked (and it's easier now with the phone apps they have to give you your security codes on the fly)
I have also seen this kind of message when someone has activated a mobile app - for example, something that would let you keep track of your AdSense earnings on a smartphone or tablet. Have you tried one of these lately? I am always wary of these, because you're basically handing over your account credentials and you don't know to whom. And if the app logs in from a different country, AdSense might count that as unusual activity.
I'd also go check all my payment and account information, to make sure my primary or secondary email, phone and/or bank information hasn't been changed.
I haven't done anything different on mobile. But come to think of it, I just purchased a Nexus 7 about a week ago and have been switching between personal and business accounts in Chrome. I wonder if that could have triggered something? Or maybe I tried logging in while already being logged into the other account. If that makes any sense.
I already check the other stuff you mentioned. It all appears to be correct.
Oh well, I just hope my account isn't flagged for something strange now. I would be on the street without Adsense. lol