Believed to be from the time when Masirah was a seaplane base, this ruin was always referred to as "The Old Guard Room," but I now believe is was a Secure Fuel Store and that is why there are bars on the windows.
You are correct, it was a fuel storage. Crews would carry a key to access it. Later, which you will know, there was a permanent base, RAF Masirah. There was a notice about the door, "RAF 1936".
This building has now collapsed completely after the more recent cyclones.
I have a website about Masirah which includes updated photograph of this building, just search for RAF Masirah Remembered.