Personally, I find it unlikely that she did wander out of her own accord for numerous reasons that I've mentioned on other threads.
For the sake of argument, it may be a remote possibility. Another possibility is that she was dumped in one by whoever took her.
Picking up on this:
We were told that on the morning of 4th May, when work resumed on the trenches, all the workers who were employed in opening the trenches, including the Engineer and the Foreman, already knew about the disappearance of an English girl from P da L, which is the reason that they carefully checked all the trenches that had been left open on the preceding night, in the eventuality that the girl could have fallen in by accident, however nothing was found.
The pics of red tubes (when were these taken?) don't correspond to manhole covers.
Did the works only concern cable pits?
Would any labourer (possibly casual) admit to forgetting to replace a storm drain cover, if ever that is what Rex Morgan noticed?
If what Morgan had seen was indeed a storm drain, where would the rainwater flow have gone? To the sea? If the engineer was only asked in mid-August, did anyone check - in early May - where any such water would have ended up? Were any police divers tasked to check?