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.
Agreed - this is a large part of the benefit of the subtle spell metamagic. Sorcerers that take it should have an advantage from being able to cast social spells in the middle of a conversation without being noticed. Otherwise, if you start doing weird hand motions and spouting off some arcane gibberish in the middle of a conversation, whoever you're talking to should either think that you are possessed or recognize that you are using magic to manipulate them and get angry.
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.
Detect Thoughts actually has great combat use in PnP - being able to know if e.g. hidden enemies are near you or anticipate what an enemy is going to do or is worried about ("I have to keep them distracted for another thirty seconds so my boss in that back room has time to get away through the secret passage behind the bookshelf! I sure hope they don't notice the button under the desk."). I've found that these kinds of things often create really great experiences in my games. I imagine they would be difficult to implement in satisfying ways in a video game, but I would be happy to be surprised about this one.
