I can't see any connection between a prior incident which might or might not have taken place and the inability to solve the Madeleine disappearance. Am I missing something Rob?
Not really, for that would be a possible reason for not divulging it, but it still should have been covered under the all encompassing "knowledge of any untoward situation involving Ocean Club users or in the village itself" statement.
There may not have been any knowledge "of any motive that could have been the cause of the Madeleine's disappearance".
To know about "motive" is much more complicated than knowing about "the untoward situation". The only way to show prior knowledge of the "the untoward situation" is to show evidence suggestive of a cover-up.
The motive is only known to the perpetrator, but the incident points to the perpetrator and without that you don't get the resolution to the case. That is the connection that I propose.
(I feel this is a strong argument.) So taking it backwards, if for sheer financial reasons the OC management decide to cover-up a prior incident and know of no motive connecting the two cases, the incident of MM going missing remains unresolved. It is not the cause of the incident but it contributes to the non-resolution.