I discussed the "here, but gone" theory back in 2005, in The Galactic Barrow's Boys. As I noted then, Karl's theory certainly refelcted our own experience with exploration:
A study of the history of human exploration shows that Karl might be on to something. Even into the first half of the nineteenth century, there were areas of the world (Africa, the Artic, the Antartic, large parts of the Pacific) that were still unknown to the Europeans. The explorations that they sent out were small, almost always under equipped, often poorly managed, and usually had no idea of what they were doing. Being British, they sometimes - but by no means always - muddled through, but almost never without mishaps. Sometimes they would visit a place, and then leave, not to return for decades.
With this is mind, perhaps Karl's theory isn't so crazy after all. What if the aliens are the galactic equivalent of the Europeans, and good old Earth the equivalent of Melville Island? Under Karl's theory, even the Roswell crash would be possible. Consider it the alien Franklin.