Referencing the headcanons that Astarion and Shadowheart would talk mad shit and gossip in Elvish around camp together, they'd catch one hint of Halsin and Bull being into each other, and they would be one of the main topics up for discussion for their next 20 conversations. They're up, doing each other’s hair, not too far from where everyone else is hanging around the campfire one night. They mumble to each other how Halsin has been blushing and smiling a lot more lately. One of them makes a stray, silly comment about how they would like to get in the middle of that sandwich, and the other seconds it before they burst into giggles.
Only they are not as far out of earshot as they think they are. Halsin smirks in their direction from across the fire and says, plain as day in Elvish, "We can hear you, you know?"
Bull can roughly understand because he has heard enough of the language at this point to put bits and pieces together. (Spy training, remember?) Safe to say, both of them are highly amused.
Either Shadowheart or Astarion replies that the point still stands, and that gets a good laugh out of Bull.