Thank you for the info, TPC! It did appear more like a drainage of some sort, but after zooming in still wasn't sure. Still wondering if it is just a water drain or what, just in case they bite into it.
Sorry I don't quite understand, but what is the concern there when a parent leaves the eggs? Could you possibly mean predators in the area, or is it that the eggs need some stones or a higher edge perhaps?
Also, IF you do want them to nest there, wouldn't you want to prepare the opening with a stone base before they return next year, so they can make a scrape.