A photograph online at Wikipedia of the Monumento a los Héroes de la Restauración (Monument to the Heroes of the Restoration) in the city of Santiago de los Caballeros very clearly shows three flags flying in front of the monument. The central flag is the national flag of the Dominican Republic, with coat of arms. To the right of the national flag is the flag of the Ministry of Culture, while the flag to the left is that of Santiago de los Caballeros.
To the left of the national flag in the photograph is a white flag with a coat of arms centered. The coat of arms features a red shield with a white border, five white seashells arranged 2-1-2 in the red portion of the shield, seven red shells in the white border, and a crown topping the shield. Unfortunately, I don't know enough background or history to understand the symbolism of the coat of arms. Randy Young, 7 May 2015
Now, regarding Santiago de los Caballeros, I believe the picture you found features the city flag (as seen here) rather than the Province flag (as seen here). Since we mentioned the same thing happened with Santo Domingo (the country's capital, where the Province was called Distrito Nacional, and has a different flag than that of the city). Here's the explanation of the CoA on their official website.
The addition of "Caballeros" to the city's name is because a group of 30 Caballeros (Knights) settled there adn kept certain priviledges in the "New World" that they held back in Spain, as mentioned in the official website here (where the full list is found): http://ayuntamientosantiagord.com/sobre-el-municipio-2/historia/ Esteban Rivera, 7 May 2015