I find that the best way to find drains is to take note of where creeks/rivers/hills/valleys are.
Due to the nature of the relationship between water and gravity, water will always try to find its own level. SO...A good general rule of thumb is "The lower the land, the larger the drain.".
I don't thing that these plots of land are left vacant for repairs of ordinary story water drains. Id be more inclined to think that they are more for extremely highly pressurized gas and water mains. The only reason I believe this is because most large drains I have been into have gone under residential areas. Think of Slyders, The Chamber, The Maze.
Take nothing but photos, leave nothing but footprints.