This marks the likely location of the tomb of Enrico Dandalo, an interesting fellow. Dandalo, at age 90 and blind, was the Venetian Doge who helped direct the sack of Constantinople by the crusaders. Our guidebook relates that after the Byzantines retook Constantinople, visitors to Hagia Sophia made a point of stopping at his tomb to spit on it.
His bones and tomb were tossed out by the Ottomans, and this marker was placed in the 1800s.