Quote Originally Posted by Logrus
Well they dident know that Luxus was part Dragon Slayer, and this dude was coming back from a 100 year old mission? (or did it only last 3 years, didn't understand that :P)
An important distinction is that Luxus was a fake dragon slayer, imbued with synthesized Lachryma. He was not trained by dragons and granted some of their power like Natsu, Gajeel and Wendy were. There was no 'part' about it, they were just copying the skills of Dragon Slayers. While it is possible, unless I am mistaken, they mentioned the practice was pioneered by Luxus' exile father. Considering he is a dark wizard, I would be surprised if he helped anyone in Fairy Tail other than his son, who himself was going to be punished by having the Lachryma forcibly ripped out of him.

(It has been a while, I'm not 100% sure on these details)

As for the 100 year mission, they claim it takes 100 years to complete, but that doesn't mean he can't come back once in a while. I don't think that they implied he had finished it yet.