Im reading Poilu and Journey to the End of the Night and they keep mentioning accidentally running into German troops/patrols(?). Another entry details Celine's character trying to find out if the Germans had captured a town or not. Wouldn't it be obvious where the line between French and German lines were? Or not?
If you look at this map of a fairly typical trench layout, you'll see that the trenches themself are not that confusing. The no man's land ranged from a couple of hundred yards to, in some extreme cases, ten to twenty yards. (Source)
On average the second line support trenches were about a hundred yards away, the third line reserve trenches another 100 to 300 yards. Of course those distances varied from battlefield to battlefield.(Source)
What made the trenches confusing was being out in the no man's land, after long artillery shelling most landmarks around the battlefield were gone. If you lost orientation it was easy to go towards the wrong trench, stumbling into your enemies lines. The closer the trenches were to each other, the easier it also was to run into an enemy patrol.
It is fair to say that the confusingness of the trenches is tied to circumstance.
"All quiet on the western front" has a good part about being desoriented in the no man's land, trying to figure out where the own trench is. The book is a good read and covers many of the questions you had.
