Questa è la risposta ufficiale delle FAQ:
Is it possible for detect magic to locate an invisible
creature?
Yes, although not very efficiently. Remember that detect
magic reveals the location of magical auras over the course of 3
rounds. A creature rendered invisible by a spell or magical
effect could be located via detect magic, but only after 3 rounds
of concentration. Furthermore, the invisible creature must
remain within the spell’s area for the entire 3 rounds of
concentration; if the creature moves out of the area, the process
must start again from the beginning. However, even if
everything works according to plan, you still don’t necessarily
know that you’ve found an invisible creature -- at best, the
caster of detect magic would know that she had located a faint
aura of illusion magic in a particular space.