My money is on the meeting between Bree and Bonnet and then the subsequent rape happening in the next episode or two.
She’ll spot the distinctive ring on the finger of none other than Stephen Bonnet, who she doesn’t know is a man to be feared when she encounters him and is talked into visiting his cabin on his ship.
There, Bonnet rapes her and returns the ring.
The replacement ring serves as a reminder of what happened to that other ring, which I take as a sign that Outlander is very soon going to revisit what happened to the first ring that Jamie gave Claire, and that does not bode well for poor Brianna.