From the article: "First it gave a shorthand description of a zone — the cathedral complex was divided into four — that read “Attic Nave Sacristy.”"

So the entire zone is called "Attic Nave Sacristy", and from that all you know is that there's apparently a fire somewhere in that zone -- could be on the floor of the nave, could be in the cathedral attic, could be somewhere in the sacristy building (even in the attic of the sacristy).

The irony is that you've made a version of the same mistake that either the security employee or the cathedral guard made -- assuming a specific location within that zone. Yours ("the nave attic") happens to be correct; one or both of them assumed it was "the attic of the sacristy building".