Related but not exactly what I'm asking: Can Thieves' Cant be understood without a level in Rogue?
Also related: Would other PCs eventually notice two rogues talking in Thieves' Cant?
Thieves' cant is a spoken coded language only shared and comprehended by thieves; as far as my understanding goes that means this language shouldn't sound like a different language, but would likely use Common with hidden messages to hide its true intention.
The situation: A rogue PC is talking to a vendor NPC who is a member of the local thieves' guild, but other PCs are in the room. While the rogue PC is able to talk in thieves' cant to the NPC, what does the remainder of the party hear during this conversation?