Not necessarily a secret mission. Picard would say they are the first to chart the place even if six ships went in there before and never came out. After all, they would not have yielded a single map for the Cartography Command.
Timo Saloniemi
Very true, and I considered that. However, I would think that any ships that were sent in that region would have made something of a map, if only as a comparison for future missions. Besides, wouldn't there be records of any ships lost in a region, before sending the Enterprise to chart it? If for no other reason than having completely informed your crew on possible dangers?