I just recently ducked into a cave as a lone Human Pikeman, looking to slay a dragon for a quest. When I met the Dragon, I realized I was too injured, and retreated to recuperate. When I returned to the cave, I went to the spot where the Dragon was, only to find a pool of blood there... a few moments later, a few messages popped up saying that the Dragon had fainted, regained consciousness, and then fainted again. Confused, I continued to search for the Dragon, finding more blood, and several dead cave-dwellers all around. Shortly thereafter, a message came saying that the Dragon had bled to death.
I searched and searched the cave, to no avail, and decided to retire the adventurer to look into Legends to figure out what had happened. Apparently, another adventurer (a wandering Human Ranger) had entered the cave, confronted the Dragon, and slain it before I could!
What nerve... Still, I guess competitive dragon-slaying is bound to happen when the work for Heroes starts to get sparse!
What the hell? Are you serious?
I went to the Travel screen, moved away, and back to heal, and then returned. Upon arriving at the cave room where I found the Dragon the first time, he was gone, and I never saw the Ranger or Dragon. The only direct evidence I saw was a lot of blood that wasn't there before, a few dead Giant Rats and Antmen, and the tail end of the Combat Announcements, as the Dragon died.
In Legends, the Dragon wasn't wounded prior to the encounter, and the Ranger's attack was in the same year as I was adventuring... I can only assume that the Ranger either came in while I was on the Travel map, or perhaps had just finished fighting the Dragon when I returned to the Cave.
Still, what a game... the wonders never cease!
EDIT: When I started my next Fortress, I got the announcement that the world had passed into the "Age of Heroes" too, which I'm assuming had to do with the slaying of said Dragon.