Also, in Niven and Pournelle's novel Footfall, the invading aliens hang out at Saturn for several years for exactly the reasons mentioned by Gary -- convenient place to resupply water and a variety of other raw materials. In their novel, the braided F ring that the Voyagers observed was that way because the alien ship had been working there at the time, and its drive exhaust had perturbed those outer rings.

So if the F ring is still braided, I guess they are still there. If they want to remain inconspicuous, after all, wouldn't it make sense to camoflague their old drive section is a beat up moon?