It would be weird if you could cast a spell in the middle of a conversation and npcs wouldn't react to it. So they'd have to implement similar reaction to how companions now react to failed attempts of tadpole mind reading, which kind of defeats the purpose of mind reading using a spell.
I'd rather detect thoughts is a cast & forget spell, that is it would keep up until the next rest. It's not like it has any combat use.
NPC's knowing you cast Detect Thoughts doesn't defeat the purpose of a mind reading spell. You find out the truth or reveal their secrets.
But a mind reading spell that always works and isn't obvious to the NPC's defeats the purpose of having the Insight (or Investigate) skill in the game.
Detect Thoughts needs to be the "blunt instrument" to find out the truth and Insight/Investigate should be the option to find out without letting them know you know. E.g. uncovering impostors with Detect Thoughts would reveal the truth and force them to attack, while successful Insight would let you pretend you don't know and set up an ambush.
