Masamune (Location of)
Radius notes that fifteen years ago, he tore up the battlefield with his other Acacia Dragoon comrades. General Viper is also noted by Karsh to have been on the mainland of Zenan years earlier. This possibly sets up the involvement of the Acacia Dragoons in the Fall of Guardia, and their presence on the mainland. If this is true, then perhaps the Masamune, after it was stolen from Guardia, ended up in the Denadoro Mts or another cave on Zenan. There, Garai and Radius found it, and Garai was killed. It then followed Garai to the Isle of the Damned.
Divine Dragoon Falls
Under this framework, Garai and Radius found the Masamune in El Nido after returning from the mainland. However, little reason exists behind the Masamune's placement in the El Nido archipelago. Nonetheless, the map used in the cutscene is that of Divine Dragoon Falls.
From: Theory (Objects)