Benjamin also isn't lost. In fact, the very phrase "10 Lost Tribes" is a misnomer. Judah, Benjamin, and Shimon all lived in the southern kingdom of Judah -- and Levi was scattered throughout all the land. However, both Benjamin and Shimon were very small and eventually assimilated into Judah, losing their seperate identity. Hence, although the Tribal members actually survived, the "Tribe" was lost as an independant entity.
The Jews of today are primarily from the Tribe of Judah (Southern Kingdom). The Northern Ten Tribes (Northern Kingdom) were prophesied to be scattered throughout the nations and to become as gentiles (not my people). Refer to Hosea 1-3.
The primary theme of the Hebrew prophets is the restoration of ALL Israel back to the Land. This requires the recovery of the Lost Ten Tribes of Israel to both their identity and their destiny as Israelites. Refer to: Jeremiah 30-31, Ezekiel 37, Hosea 1-3, and Isaiah 11
Is this your understanding?