The NPC is someone they've long sought and he does indeed work for Mr. Redbeard, and would not want that known. The two are up to some dastardly no good together and those meddling heroes could ruin it.
That really doesn't give me much to go on. I presume you're looking for a more general answer about how we use
detect thoughts in our own games, rather than advice specific to your scenario?
Here's an example from my old game:
Bard PC used
detect thoughts to gauge what a guard leading him to a secret meeting with the queen was thinking about, suspecting this particular guard after a sour interaction between guard and other PCs.
This guard was torn about recent shake up among the royal guard, as several of his brethren were mustering behind the banner of a new commander loyal to the queen's evil son who sought the throne even as his father lay dying. On one hand, guard had sworn to follow his commander and viewed the dowager queen as an unsuitable ruler (cause prejudice / realpolitik paradigm), but on the other hand he genuinely despised the evil prince-who-would-be-king.
Secretly, the guard was leading the bard PC into a trap. The evil prince learned about the secret meetings the PC was having with his mother and sent a false message to lure the PC into the trap.
Bard PC guided conversation towards the guard's opinion on politics for the throne, trying to suss out whether the guard was friend or foe.
I narrated something like this for his surface thoughts:
"A soft indecisive queen or a treacherous murderous heir? I'd rather not choose at all. Asking me like I have a choice at all. Mine is to serve and I'm sworn to serve Commander Valense. Perhaps this sycophant deserves the blow back from the high council that is coming his way."
Even with the leading question, it wasn't enough to give an obvious answer about which side this NPC guard supported, nor was it obvious that there was a threat. However, at the time if the player had read into just a little more, been just a little more alert, he might have picked up on some (very subtle) clues I was dropping:
- At that time, the players strongly suspected Prince Ardein had murdered a couple people, but that wasn't common knowledge. In fact, the "murderous" part of his surface thoughts had to do with the trap he was leading the bard PC into.
- The players previously knew that Commander Valense had sent his wife to stay at Prince Ardein's summer estate, and they saw him visit the prince's box during the Tourney of Flowers, so I wasn't exactly hiding that Valense had become loyal to Prince Ardein.
- The "blow back" from the High Council could refer to a previous game session where the High Council denied a motion to censure (initiated by PCs and their allies) Prince Ardein, however that was kind of above this guard's pay grade. This hinted that he'd been led to believe that the trap for the bard PC came from the High Council.
Anyhow, the player didn't catch the subtle cues gained from surface
detect thoughts, and didn't want to probe deeper. He got ambushed, used a reaction invisibility power and fled for his life. After the fact, he started connecting the dots and groaned.