I don't know, my players asked me this very question yesterday.
I agree with this. As written, I think it doesn't detect active spells.
On the other hand. In The Red Book of Magic, page 105 "How Does Spirit Magic Appear, Sound and Feel?" says "It's is noticeable when spirit magic has been cast. For example a sword with Bladesharp cast upon it might have a supernatural sheen or a person with Protection cast might glow dully".
I don't know what to make out of it. Is there any other method to identify individual active spells on a target?
Sorry for the necrothread!