No review copies had been sent out; the contents had been kept a closely guarded secret.
Times, Sunday Times (2014)
The film's working title is a closely guarded secret.
Times, Sunday Times (2011)
The scale of the rewards is normally a closely guarded secret.
Times, Sunday Times (2015)
The honeymoon destination remains a closely guarded secret.
Times, Sunday Times (2011)
These reports are a guarded secret between the production staff.
Times, Sunday Times (2010)