Via Fred at headsup, this gem from the folks at AP Stylebook (larger version here, proving which page it was on).
So, "his whereabouts is unknown" but "their whereabouts are unknown". Because, as Fred says, "his house is large but our house are large".
And yes, you can use either "is" or "are" with "whereabouts", but pretending it has anything to do with the number of people or things whose whereabouts you're discussing proves you don't actually know anything about the word or, probably, the language.
Then again, this is the AP Stylebook.