To the end of the south of the apse there is the diaconicon. Wikipedia: “The diaconicon (Greek: διακονικόν, translit. diakonikon; Slavonic: diakonik) is, in Eastern Orthodox and Eastern Catholic churches, the name given to a chamber on the south side of the central apse of the church, where the vestments, books, etc., that are used in the Divine Services of the church are kept (the sacred vessels are kept in the Prothesis, which is on the north side of the sanctuary). The diaconicon contains the thalassidion (piscina), a sink that drains into an honorable place where liquids such as the water used to wash holy things may be poured, and where the clergy may wash their hands before serving the Divine Liturgy.”

On two pictures one sees what I took to be a grave. It may well be the thalassidion instead.