Who is buried in Serra's tomb??

Hint; it is not Junipero Serra.

Actually no one is buried in this tomb.

Junipero Serra died in 1784 and was interred under the floor of the main altar "on the Gospel side." (The left side from the congregations perspective)  In 1924 this Cenotaph was dedicated with the thought that Serra's remains would be interred in it. Public sentiment was overwhelmingly opposed to removing Serra's remains from under the altar.
The monument was placed in a side chapel at the mission. The figures represent the four monks who are buried beneath the altar. Father Serra, Father Crespi, Father Lasuen and Father Lopez.

The Death of Serra