As I stated in my presentation at the Spanish congress of vexillology on November 1998 - the Nicaraguan autonomous regions of RAAN and RAAS (Regiones Autonomistas - Atlántico Norte and Atlántico Sur) didn't officially adopted a flag.
Jaume Ollé, 4 February 2000

No regional flags are visible. We have a photo of the Bluefields airport where there was was hoisted a Nicaraguan flag and an unknown white flag with arms. I am not sure if that is the flag of RAAS. The Coat of Arms are only possible and subject to further confirmation.
Source: Elisabet Ollé personal observation.
Jaume Ollé, 5 April 2008

The flag of RAAS - Region Autonoma del Atlantico Sur can be seen here. A photo of the flag can be seen at this blog.
Valentin Poposki and Paraskevas Renesis, 4 July 2009

Here the logo and the flag of RAAS from presidencia web site.
Jens Pattke, 4 July 2009

Coat of Arms

image by Jaume Ollé, 5 April 2008