Hidden Lairs are mini-dungeons that contain a Purple Rare Elite at the end. It has a 100% droprate of a random normal gem. However, there is a chance that upon killing the Purple Rare Elite that an “Explore Further” portal opens.
This portal allows you to earn a second normal gem by killing the second Purple Rare Elite.
However, how do Hidden Lairs spawn? I have found through hours of gameplay and observation that shrines and treasure chest variants have a % chance to respawn a Hidden Lair.
The maps I have charted all reveal that Hidden Lairs respawn near the location of shrines or treasure chests. So after seeing this pattern in my maps, I tested the theory. They trigger the respawn of Hidden Lairs, Bonus Quests, and Purple Rare Elites.
These example show you a correlation between where a Hidden Lair respawns, and what triggers their respawn.