I can't actually think of any examples of myth-chasing plots in fantasy or sci-fi off the top of my head, unless you include the vast number of fantasy stories which feature prophecies.I clearly wasn't thinking too hard about the sci-fi, because there's one obvious myth-chasing plot that turns up all the time. It's based on the Fermi Paradox, which basically goes like this: given the size of the universe, and even moderate probabilities for the emergence of intelligent life, where are all the aliens?
There's a huge amount of sci-fi on this topic, although a lot of it doesn't really qualify as myth-chasing. In David Brin's Uplift series, for example, the explanation for the Paradox forms the basis of the setting, but it isn't really a mystery. Sometimes, though, it's all about trying to chase down the solution to that puzzle. Alastair Reynolds' Revelation Space series is an excellent example, and perhaps that's why I like it so much. My recollection is that first book in the series -- Revelation Space  -- even begins on an archaeological dig.
Oh, hey, look: Jo Walton wrote an article on the Fermi Paradox in science fiction for Tor.com. So there you go.
I've also been pondering the possibility that the Big Dumb Object story is somewhat related to the myth-chasing plot. There are obvious differences -- the Big Dumb Object isn't exactly something you have trouble finding! But the process of unravelling its purpose, and how it got from there (wherever there is) to here, is not entirely dissimilar to piecing together the truth behind a myth. Maybe that's a bit of a stretch, though. What do you think?