If it is plain apostles found everywhere as decoration maybe not. if it's a more unique and notable subject (say it depicts a notable historic person, ie. some person the castle was originally made for) or has some story to it, there may be reason to map it.

Shila-na-gigues, reliefs and moisaics are usually unique/historic enough, for example. As are statues standing inset into the walls of touristic buildings in Italy.

If it's purely decoration and some random anonymous face it's unlikely to be worth mentioning. (Except if some respectable authority considers it of outstanding/heritage quality for some reason or another.)