Consider the Night Hag. The Night Hag has "constant detect chaos, detect evil, detect good, detect law, and detect magic."
However, each spell reveals additional information for each round "you study a particular area or subject." Detect Magic, for example:
1st Round: Presence or absence of magical auras.
2nd Round: Number of different magical auras and the power of the most potent aura.
3rd Round: The strength and location of each aura. If the items or creatures bearing the auras are in line of sight, you can make Knowledge (arcana) skill checks to determine the school of magic involved in each. (Make one check per aura: DC 15 + spell level, or 15 + 1/2 caster level for a nonspell effect.) If the aura emanates from a magic item, you can attempt to identify its properties (see Spellcraft).
Would a rogue wearing magic items be able to sneak up on an inattentive Night Hag? (Assuming his Stealth is sufficient.)
Unlike continuous True Seeing, which is entirely passive, Detect spells seem to need to be activated. In other words, does a creature with continuous Detect Magic need to consciously choose to begin using the ability before it has any effect? If not, how do you handle the fact that the area of the spell is a cone, and not a radius? And if so, what is the difference between "continuous" Detect Magic and using it "at will"?
(Unless something in Pathfinder overrules it, D&D 3.5e answers are good too.)