Bono, if you look at the grey underlay you will see that it shows track coverage of the whole area but with gaps.
It is standard search procedure to set out post-flight the actual areas searched. Subsequent searches will thentry and cover the gaps.
The yellow overlay is the total area and when that is searched further gaps will appear.
Now we know in this case that the geographical location of those gaps moves hour by hour so really it is luck if you don't miss any on the refly.