Auras
Auras are treated as having a duration of 1 round and affect a creature each round they are in the aura or move through it (willingly or not). They allow saving throws to negate every round (unless it deals damage, then its a saving throw for half). Anti-paladin auras, for example, affect a creature immediately upon them entering it (willingly or not) and allow a Will save DC10 + 1/2 antipaladin's level + antipaladin's Charisma modifier to negate the effect each round. A fail means they are affected for 1 round.

Active Use of Stealth: You must make a new Stealth check as a move or part of a move action every round to actively use Stealth. If you do not actively use Stealth but are still hidden in some way (cover, burrowing, invisible), treat the Perception DC to spot you as -10 + Stealth bonus + other modifiers. In any case there there is a given Perception DC to spot something that is not actively using Stealth, assume this is the case instead of the normal DCs given the Stealth skill description.
For example, detecting a burrowing creature is not DC25, but DC -10 + 10/foot of thickness + active Stealth. So a creature directly beneath you would have at least 1 foot of thickness (10 -10 = 0) and be DC0 + Stealth bonus whereas a creature 1 space below you would have (50-10= 40) DC40 + Stealth bonus. Of course, a creature with Tremorsense, Blind Sense, or Blind Sight would have no problem detecting a burrowing  creature within range, no Perception necessary.