[Someone]

Certainly with the BBC, that likely means "we talked to people who gave us information we judge to be correct, but we cannot tell you who those people are (likely because that was a precondition for them giving the information)"

See http://www.bbc.co.uk/guidelines/editorialguidelines/edguide/... (I would make a clearer distinction between unnamed ("we know who (s)he is, but cannot tell you") and anonymous ("we don't know who (s)he is, so we cannot tell you") sources)