The PCs took normal precautions and saw the dragon before it saw them. It's not exactly hard. Dragons are big. What's your point?
My point is that your PCs entered a dragon's territory, discovered its existence, and got away safely. Though dragons might be big, they are also fast and once the PCs are seen, it is entirely up to the dragon whether there's an encounter or not.
So your PCs saw the dragon before it saw them, and I assume it didn't see them at all.
Is it impossible in your sandbox for a black dragon to spot humanoids making a stealthy escape? How about if they're taking "normal precautions" (ie. before they themselves saw said dragon?)
So the real question is, if it's
not impossible, in your sandbox, for a dragon to discover humanoid creatures merely taking "normal precautions" in its territory and then decide to kill them regardless of their words or actions, is it only impossible for this to happen to player characters?
According to your sandbox's rules-of-physics-and-whatnot, was there any chance that your PCs could have, unknowingly, passed a point-of-no-return in the game, where a TPK was inevitable based on average dice rolls?
I'm sure you know where I'm going with this.
The trick to running a very good sandbox campaign is to trick the players into thinking that they're playing in a very good sandbox campaign.
...most of DMing is like that. How do you create a vibrant, living, multifaceted gameworld? Well, you trick the players into thinking that they're in a vibrant, living, multifaceted gameworld.
So true.
And I daresay that anybody
claiming to run a legitimate sandbox is not only deluding their players, but themselves as well.
A true sandbox game would be an insanely frustrating experience.
I've seen a lot of games where the DM let world-building trump fun...and games where terribly un-fun circumstance were justified by making the world more realistic...none of them ended well (including those of this ilk that I've run in the past).
Every single time I've experienced a game where the details of the setting are written in stone - ie. can't be modified on the fly to fit the adventure at hand - I have found world-building to trump the fun.
Which is why I'm wary of anybody who is a world-builder first and a DM second - they have so much emotional investment in their
work of art that the other people around the table have to take a backseat.