Reading the exact text, I think Detect Magic would detect the aura. It doesn't specifically list it in the spell. You could maybe try to cram a RAI argument in there, but as a DM, I'd allow it.
Also, how about simply throwing flour or some other powder on the creature? It explicitly states that mundane conditions can render the creature detectable, but also explicitly forbids Dust of Appearance and Glitterdust, which function similarly. Still, so far as I can tell, if the already invisible person picks up an item, the item would still be visible unless stowed, so in theory, the flour would be visible on the surface.
BTW, what book is this spell in?