I started D3 and logged into the game. My level 60(7) Barbarian was selected and I quick-joined a game with a level 60(4) character. Only the two of us were in the game. After some game-play, I had 4 or 5 Nephalem Valor (NV). I then returned to town and exited the game.
A while later (possibly less than 30 minutes), I started the game again, logged in and quick-joined the same party, still with only the two of us present. I don't recall my NV buff at this point. I bannered to the other player just as an elite was being killed I think. A minute or so later I noticed I had 5 NV stacks, with 28 minutes remaining. The only explanation is that NV stack persist across exiting the game?
After playing further in the party, I left the game and the party and joined a normal public game. At this point, there were no NV stacks.