His whereabouts is unknown
His whereabouts are unknown
Which is correct, or is this simply a matter of preference?
Wiktionary marks whereabouts as plurale tantum. Merriam-Webster says that it's a "noun plural but singular or plural in construction". The Collins English Dictionary says that it is "functioning as singular or plural", and the American Heritage Dictionary of the English Language says that it is "used with a singular or plural verb". The Corpus of Contemporary American English has 23 cites for "whereabouts are unknown", but only 3 cites for "whereabouts is unknown". Finally, Google returns 1.1M results for "whereabouts are unknown" vs 191k results for "whereabouts is unknown". So, neither of the forms is unheard of, but "whereabouts are unknown" is preferred by a rather significant margin.
Since @RegDwight has already provided an answer with strict supporting background information, let me answer this question from a conversational perspective.
I would think of a person's whereabouts as places where he has been. Note my emphasis on places. That's plural. So,
The context here would be that we do not know the places he has been going to.
Surely whereabouts means 'Where he is or which place he is in' and therefore the singular is correct i.e. His whereabouts is unknown. We wouldn't say 'Where he is are unknown'. He can only be in one place.
Collins dictionary states that whereabouts (noun) functions as singular and means the approximate place where a person or thing is.
Just because a lot of people say 'whereabouts are' doesn't mean it is correct! They look at the 's' and assume it is plural.
The Associated Press 2011 Stylebook records "whereabouts" to take a singular verb.
"His whereabouts is a mystery."
While I believe it to sound and appear incorrect, the AP is the horse's mouth.
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