Camino Photo of the Day: Sacristy Door?

This door in the Catedral de Santa María de Astorga is conspicuous by its inconspicuousness. Where does it lead? I suspect it leads to a sacristy, based solely on the fact that there is a thurible stand outside. But it may, in fact, be a storage closet, or something else entirely. In any case, there does not appear to be any way inside, except by key.

Date: 20 April 2016
Place: Astorga (Castilla y León, Spain)