Blocked sign-in attempt to Google Account

This morning I had an email in my inbox from Google saying "We recently blocked a sign-in attempt to your Google Account (Date & Time: Saturday, January 10, 2015 3:01:26 AM UTC Location: Oxford, UK)"

They provide a link: http://ift.tt/13ZSu8M

This takes me to the following:

Originally it showed me the IP of the attack. I then put "what is my IP" into Google to discover that it is actually my own IP, so I clicked the "this is actually me" box, and hence I can no longer show the screenshot with the IP.

My question is: what could have caused this? I went to bed before midnight; I think I closed the lid last night (in which case the machine should have been in sleep mode), but even if I didn't -- I usually leave it on some radio station when I sleep -- it hasn't given me any warnings before, so why now?

The page says it isn't my Mac that is causing the problem. But the only other device I have is my android phone and the Wi-Fi is disabled, and it doesn't have any 3G type Internet. I keep those things off to save battery. It did have Bluetooth on (I must have forgotten to turn that off days back).

I've also got a standard Apple issue keyboard and trackpad, and this morning the trackpad bleated about wanting new batteries.

That's it! That's all the electronic things I have. And a wi-Fi repeater which plugs into the downstairs router via an Ethernet cable.

So what could this warning be? Someone else is attempting to sign into my account from my home IP? That's really unlikely. It is a family home, I doubt one of my family is trying to hack me, I would however be impressed if this were the case. I haven't handed down any old computer equipment either.

So I have no clue.

Can anyone think of any possible explanation?