Betting you wound up falling through a hole in the map.
Clicking on the linked image above will take you off the City of Heroes site. However, the guides will be linked back here.
Solo'd this one time, and one of the Ancestor Spirits with him wound up in this spot. I called a GM and he teleported me to the bad guy. Had a heck of a time finding my way back out.
Ok just giving you all localisation of a very hidden place that I absolutely found by accident:
In the mission given by Doctor Dream in Dark Astoria: Entitled Defeat Tub-Ci.
In the moment I arrived at the last room where the AV is standing I casted incandescence on the exact place where he stands and then... I was falling down to a very long tunnel that bring me to a large cave which has been specificaly designed (it's like bat-cave but it's Tub-Ci-Cav ^^), there are some stair to go back upstair even if it's no way to go back unless using incandescence on the very close top of the cave, there is green chemical water there and some left tonnels with dead heads drawn. I am pretty sure that there is something else to discover in that place that is VERY large and high but I couldn't find unfortunately...
If any have some such hidden places to share please post.
Chaos War God, xX-Chaos-Team-Xx
Satan's friend, Death Dealer