Actually,I had noticed it,but I didnt paied enough attention...
Anyway,the dragons dissapeared at the year 777,but we saw Igneel!It was something like a ball talking to Igneel for Natsu(?,I cant remember precisly and cant remember in which chapter was it)...And also Gildarts Clive gave him some hint about Igneel!
I surfed a while in Fairy Tail Wiki,to search what he told and I found that:"As Natsu arrives at his house, he instantly asks his relationship with Lisanna seemingly still unaware that she supposedly died two years ago, which bothers Natsu. As Natsu attempts to leave, he tells Natsu that he met a Black Dragon which captures Natsu's attention. In his mission, Gildarts explains that the Black Dragon attacked him which resulted in him losing his left arm, a leg and an organ, which was also the cause of him failing the mission. Gildarts warns Natsu how a human cannot stand up to that dragon but Natsu says thats what Dragon Slayers are there for and that he may have a chance. Natsu rushes off in hope that the Black Dragon will help him find Igneel, and Gildarts encourages Happy to support him all the way. Gildarts was most likely sucked into the Anima along with the rest of the town, though his Edolas counter-part was not seen. ">>>So he saw dragons!A black one!Or it wasnt?And it was sth different than the oiginal dragons....And this was a "task" or sth for his 10-year request,that he,the strongest member of Fairy Tail,stronger than Makrov,as himself says that,this request it was a 100-years request!
I dont have any theory about how dragons dissapeared or sth...But I think that may have connection with another "world" not earthland not Edolas but another one,different!I dont know precisly,and these info's where the best i can do...