One NPC casts a detect thoughts spell before meeting with the PC. Once with the PC, he starts using detect thoughts to read the minds of the PC around him.
One of the PCs happens to have a detect magic spell active and is concentrating on the area.
What will the detect magic spell reveal? Will the NPC show a magical aura due to the functioning detect thoughts spell?
Detect thoughts is a spell with a school and duration like any other. I see no textual reference why it would not be detectable by detect magic, which should reveal presence of a magic aura, then number, then strength, location and school (Divination, on a spellcraft check) as normal.
Note, when the caster casts the spell is what's important. The caster would be concentrating the entire time on maintaining the spell, regardless if they are probing thoughts. I think the word "use" here may be the cause of your confusion. Consider: "Presence or absence of magic".
