Children who fall into drains and come to harm are usually eventually found as happened in Aisling's case.
The majority of children who fall into storm drains or sewers are recovered relatively unharmed ... unless the fall happens at a time when there is a torrent of water which washes the child away.
Even when there is water in the drain children have been known to survive long enough to be rescued ... and those who have been washed away are usually discovered at the drain outlet.
In Praia da Luz the drain outlets appear to be on the beach.
We are told there was a thorough search carried out for Madeleine McCann; if that is so, the drains will have been an obvious place to look either for a child who had had an accidental fall or for a child whose body had been deposited therein.
I do not think that Aisling and Madeleine's cases are compatible ... I do not think Madeleine was or is in a drain in PDL.