The Sultanate of Tadjoura was one of small sultanates located on the African oast along the Red Sea as far back as the 12th century. In 1862, an envoy of the Dardar or Afar sultan allowed the French to use the port of Tadjoura. His flag was plain red, also common among many states along the Arab peninsula.

The chronicles mentioned four small Sultanates which controlled the caravan traffic with Ethiopia.  Tadjoura was one of them. The Sultan of Tadjoura was called "Dardar". His power was represented by two "sacred drums", which were buried for one year after the Sultans death. The Dardar of Tadjoura accepted a British Protectorate in 1840 and the Musha Island, in the Gulf of Tadjoura, was given to the United Kingdom.  In 1862, an envoy of the Dardar signed in Paris a treaty allowing the French vessels to moor in the port of Tadjoura. Tadjoura became an important port of call for the vessels sailing to Madagascar and Indochina, which were not allowed to moor in Aden following British colonization in 1839.  The Sultanate was later incorporated into the Republic of Djibouti.
The flag was a 1:2 plain red flag.

The red colour of this flag as being associated with the Red Sea. 
However, red flags were used by Muslim countries throughout North Africa and Arabia, not only (and absolutely independent to) those used in Oman and Zanzibar.

An Afar girl from the Sultanate of Tadjoura wears exotic gold jewellery for marriage

Djibouti, Tadjoura. During a dance, Muslim girls from the Sultanate of Tadjoura, dress up in all their finery