I found this interesting: AlphaGo's internal statistics predicted victory with high confidence at about three hours into the game (Lee Sedol resigned at about three and a half hours):

For me, the key moment came when I saw Hassabis passing his iPhone to other Google executives in our VIP room, some three hours into the game. From their smiles, you knew straight away that they were pretty sure they were winning – although the experts providing live public commentary on the match weren’t clear on the matter, and remained confused up to the end of the game just before Lee resigned.

Hassabis’s certainty came from Google’s technical team, who pore over AlphaGo’s evaluation of its position, information that isn’t publicly available. I’d been asking Silver how AlphaGo saw the game going, and he’d already whispered back: “It’s looking good”.

And I realised I had a lump in my throat. From that point on, it was crushing for me to watch Lee’s struggle.

Towards the end of the match, Michael Redmond, an American commentator who is the only westerner to reach the top rank of 9 dan pro, said the game was still “very close”. But Hassabis was frowning and shaking his head – he knew that AlphaGo was definitely winning. And then Lee resigned, three and a half hours in.

Also this bit, suggesting that Lee might still win some matches:

Silver said that – judging from the statistics he’d seen when sitting in Google’s technical room – “Lee Sedol pushed AlphaGo to its limits”.