I have two PCs in the party that know Thieves' Cant and they constantly talk to each other and pass secrete messages to each other via Thieves' Cant. Another player thought that because Thieves' Cant is being used so often he should have a chance to recognize that something more is being said, he has not suggested he would know the hidden message but that perhaps notice that a message or something was being added to the normal conversation.
The PHB states:
… a secret mix of dialect, jargon and code allows you to hide messages in seemingly normal conversation. Only another creature that knows thieves' cant understands such messages. It takes four times longer to convey such a message than it does to speak the same idea plainly.
His reasoning is that mixing in the jargon, dialect, etc would make the conversations seem odd and allow him to start figuring out they are not normal conversations after a while. He also stated that Thieves' Cant can only be understood by the listener if they were clued in that there was a secret message but he did not provide any grounds for it.
Is he right on these two points or is Thieves' Cant done in such a way that anyone knowing it will pickup the message with out prompt and anyone listening in over and over on multiple conversations would never pick it up?
Best Answer
No, because the point of Thieves' Cant is that observers can't tell it's being used. The conversations being lengthy isn't a giveaway either, since that just means that the conversations the messages hide in are normal length and the Cant message is ¼ the length of the carrier message. For longer Cant messages, just talk more about mundane things like your sister's guild training or that horse you were admiring in the last town.
But even if we assume (or it's been house-ruled) that a listener can tell when Cant is and isn't being used, just not what the message is…
Still no, not if they're using Thieves' Cant properly. To use Thieves' Cant properly, you have to use it all the time with your fellow rogues, just as part of normal casual conversations with them, so that even if someone twigs that the conversations are “not normal”, you don't give away why you're using Cant by presenting an obvious pattern. If the only time a pair of rogues use Thieves' Cant is when they're up to no good, even the most Lawful Stupid paladin can figure out that “unusual conversations = I should start interfering!”
When it's used all the time between the two rogues, even someone who has learned to recognise the use of Cant won't be able to tell that they're up to mischief. It's definitely not grounds for making intuition checks or anything to figure out that something is up. When every conversation between the rogues is “not normal”, then it stops being suspicious.
Part of the point of Thieves' Cant is that knowing that there's a secret meaning is not the same thing as knowing that there's an important secret meaning. Two rogues bantering using Cant could as easily be talking about inconsequential things — ribbing each other, cracking jokes, discussing last night's hangover — as they could be talking about real plans. When all the talk between two rogues is laced with impenetrable jargon and slang, nobody can tell when to be concerned and when it's innocent.
The same principle is observed in modern cryptography — if you only encrypt sensitive messages, then anyone monitoring you knows when you're discussing sensitive topics and can probably figure out the message based on context, like time, other activity, and receiver.
Thieves who speak in Thieves' Cant only when they're discussing doing something improper aren't very good at being thieves.