SaturnV;
Thanks for the interesting link.
We can only assume that the COSMO satellites failed to detect further signs of the "Pollution Spot" on later passes. That being the case, the substance had either evaporated or had become broken up by sea/wind action to such an extent that the remaining areas were less than the 30m by 30m SAR resolution. That could also be an indicator that it was a hydrocarbon distillate, and could have been kerosene.
In an earlier post, I said that there were 3 COSMO satellites in orbit in 2009, and assumed that they were orbiting in the same plane, i.e. following each other 120 degrees apart. That may not have been the case, and if the polar orbits were in independent planes to each other, there is a chance that there were gaps near the equator in the satellite coverage.
Don't forget, it may have been an algae bloom.
mm43