So I can run a lair, leave and go back in immediately and sometimes the lair is normal and enemies are present sometimes the lair is empty.
I've had an empty lair after changing characters and going in with a different character...
I've had an empty lair after a 5 minute wait on the same character....
Obsidian Moonlight - Paladin
Obsidian Oath - Warlock
A whole lot of other Obsidian toons as well.