You could have them go through a ledger or other written notes that could pertain to the prisoners, they could either slice through the captors, bargain/scheme for the information, search for the list and look through it before they get caught -or steal it, I assume they might forget what it says otherwise- Most prisoner lists back then would just be the name of a prisoner with a short description of the prisoner; their facial features, tattoos, body style, race, etc. Although I am genuinely surprised that they just up and left not caring about knowing who they were looking for, however, as a DM you can go pretty much anywhere with this situation. Force them to purchase his freedom, accidentally kill him and have to find a replacement or revive him, force them to break ties with the captors when the captors start getting rid of their prisoners -sold, moved, executed-
If you feel like being a little mean have them jump more hurdles by putting them in situations that are much harder/time consuming.