Detect Magic & Invisibility

I’ve got a question on detecting invisible creatures/objects via Detect Magic.

So the spell does not detect invisible creatures/objects, but the magic itself is seen as an amorphous glowing fog.

Will the caster “know” this to be an invisibility effect? Or is it reasonable to assume that it’s just a magic aura of some sort, being one of many possible types of magical auras.

I’m leaning toward the latter, as I don’t think the spell lets the caster know what type of magic a creature/object is subject to… but only its presence.

That's correct. It just detects an amorphous glowing magical aura. 

Thanks much Alex.