I have version 1.15. I played Babylonians on a big map. At 1340 AD I knew 3 other civs (and a fourth I had erradicated). Then, suddenly at 1345 AD, without any obvious reason, the remaining 3 civs popped up in the diplomacy screen. I hadn't traded world maps or contacs or anything and didn't see any of their units. They just popped up. This was first time I clearly took notice of this but I think it's happened before. Seems like a bug to me...