practicalm
Adventurer
I was pretty sure there was an adventure in a Dungeon Magazine where the map was inside of a sphere. But I wasn't able to find which issue it was in.
Given that the scenario proposed in the OP is specifically that of a dungeon, the sphere's "surface" is here just the dungeon floor, I think; there's still a ceiling and who knows what else above the rooms/passages/etc. the PCs are to be exploring.The dungeon looks suspiciously like the Death Star. In particular whenever you go to a lower level.
If the PCs are supposed to be on the surface of the sphere, then the PCs should see a too-close horizon. Running along a straight flat passageway means you go 'over the horizon' from your starting point.