There's a number of bird types that have 90.0 Detect Hidden skill, and will passively reveal stealthers. Ravens, Crows, and Turkeys. Why it's always been a pain in the ass stealthing around cemeteries, Ravens and Crows like to spawn and hang out around there.
Squirrels have 70.0 Detect Hidden. So beware of them when stealthing around in the Labyrinth.