Some creatures of have Blindsense, an ability that lets the creature notice things it cannot see but without any precision. The creature with blindsense usually does not need to make Perception checks to notice and locate creatures within range of its blindsense ability, provided that it has line of effect to that creature. Blindsense allows the creature to sense that opponents are nearby but does not actually allow it to see them.
Any opponent the creature cannot see has total concealment (50% miss chance) against the creature with blindsense, and the blindsensing creature still has the normal miss chance when attacking foes that have concealment. Visibility still affects the movement of a creature with blindsense.
A creature with blindsense is still flat-footed against attacks from creatures it cannot see.