Attic nave sacristy is a location
Sacristy seems to give the location of where in the nave, which considering that the cathedral is cardinally aligned, is a rather poor choice.
"Attic nave south" would have been a perfect description.
The real problem was that both the main cathedral and the sacristy (a separate building adjacent to the cathedral) had attics; either the security employee got confused and told the cathedral guard to check the sacristy's attic, or the guard got confused and decided to do that.