...stuff comes OUT of wild dungeons, right? To, you know, feed on stuff, like merchants stupid enough to be hanging out too close.And I thought it was an interesting enough question to bring it into the light of day and answer it as a post of its own.
The short answer is, no. The only things that come out of megadungeons (such as Castle Greyhawk, Maure Castle, Castle Blackmoor, etc.) are treasure and adventurers, some of the latter on their shields rather than holding them.
The reasons for this are manifold. The geography of the megadungeon is definitely a factor against it. Most traditional megadungeons have choke-points in the dungeon itself that serve to restrict movement from level to level (or, alternatively, region to region). In most cases, these are stairs. There are just so many staircases that lead from one level to another, and in many cases there's only a single staircase that even breaches the surface. (Castle of the Mad Archmage, I should point out, has a number of such entrances, including some that lead directly to some of the deeper levels of the dungeons, as did Castle Greyhawk which inspired it; but the principle holds, as those are still limited in number.) Those trolls on level 10 of the dungeon just aren't going to spend hours schlepping up through the upper nine levels to get to the surface. What would be their motivation? Food? Loot? If that was what they were looking for, they wouldn't have put their lair 200' below the surface of the earth, with hundreds of traps, tricks, portals, and other monsters between them and daylight. Those dungeon denizens are there to stay.
Too, it should be remembered that, not only is the megadungeon a "mythic underworld" in the classic sense, but it is also, for the most part, a closed ecosystem, fed occasionally by protein from adventuring parties and the odd influx of gold from insane wizards. That's Gygaxian naturalism at its best; while it may not work out entirely square in terms of the ratio of large carnivores to herbivores, at least there is usually an attempt to have sources of water, lichen eaten by vermin eaten by small things eaten by big things, etc. The large nasties that would rampage across the countryside simply have no incentive to do so. They made their lair in that maze because there was an ample supply of giant rats, not because of its proximity to deer (or humans) on the surface.
Note that this is all said in relation to the megadungeon. A smaller dungeon, or a monster lair, is a different thing entirely. In those environments, most of the time the big nasties are going to come out, if for no other reason than it's easy to do so and the underground isn't large enough to support enough giant rats and centipedes to sustain a clan of ogres and trolls.
So the environment around a megadungeon is probably going to be relatively safe. Getting out is relatively difficult (just as difficult as getting in), and a megadungeon by definition is going to be large enough to be able to sustain the creatures within without forcing them outside for food. Creature lairs and small ten-room dungeons are a very different story, but nobody is talking about setting up an inn outside the entrance to a lost demon shrine. Those cultists are lousy tippers anyway.