Issues with the 'opponent might have left' feature in classical chess

Today I was playing a game with 15+0 time control. The link for which is below.
http://en.lichess.org/UfOgDX3p/black
My opponent was offline for about 5 minutes ( he had more than 10 minutes before his last move and all this time he was offline). I didnt get any 'opponent left' message from lichess.
I waited out and he came back with a move after 5 minutes. It took me 3 minutes to know that he made the move. But within that time he could claim vicotry with 'opponent left' option.
Why didnt I get that option when he was offline for more than 5 minutes but he gets it within 3 minutes?
Anyone can throw some light on this issue?

Well as long as you have the game open in another tab that can't happen. As far as why you didn't ge the option, I'm not sure. Sometimes you get it within 30 secs and sometimes you have to wait a few mins. If anything they need to make it consistent.

I get it, sometimes the other persons net does go out. They need time to reconnect, but it should be consistent.