Thanks, everyone! Lots of good information here. Customized mazes was something I never considered, but in hindsight it's the obvious explanation. It's pretty cool/useful for gaming, too.
I will say, though, that I think "as secure as the DM needs them to be" isn't usually a good answer, because it's the underlying assumption about everything in D&D: As long as the players don't revolt, the DM can make everythign work however he likes. But while a DM can make all these decisions ad hoc, the reason I ask the question is that I want to know other people's views and/or the canon answer, both for information (don't want to ad-hoc), inspiration (what intriguing features or loopholes do the various explanations have) and simply to know what I'm deviating from and what changes I'd need to make, if I decided a maze needed a spesific feature for plot reasons.