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.
Not necessarily. There's always the ethical dilemna of reading others' thoughts or controlling their mind. Someone playing a character who thinks it's morally wrong to read someone else's mind without their permission would likely rely on other means to accomplish their goal, even if it was more difficult.
However, if caught, I agree there should be greater consequence for using detect thoughts. Most people are not going to be like "You ready my body language?? How DARE you!" and attack. But that would be an understandable (and likely) response if they knew you read their mind.
I definitely think it should have a chance of failing too (perhaps a perception check), even if it's an "always on" spell or inherent ability. It's kind of like pickpocketing someone's mind.
