Success was made possible by the fact that they were dealing with maybe the least observant dragon in the whole history of dragoning. Over the course of the evening, it got to make maybe twenty perception rolls of various sorts; none of them scored more than a seven. And whenever its very respectable passive perception might have been relevant, the party were happily rolling natural twenties for their stealth skill as if there was no other number scribed on the die. (There was, I checked.)
All that was to the good of course, from the players' point of view, since if the dragon had actually found them it would very probably have made pretty short work of the whole lot of them. As it is, they nicked its loot, torched its lair, and got away unscathed.
I have the worst luck with dragons.
P.S. Annette was most aggrieved that I didn't mention that they were clever too. So then, they were clever too.