Is it the one you have to go to Redcliff for? I know there's a smuggler there, but I don't remember if Cullen was involved in that quest in particular. Does the journal tell you anything when you look the quest up or not?

I've seen and recruited the smuggler in Redcliff. The journal just tells me that Cullen has asked me to find letters from or about (can't remember the exact wording) lyrium smugglers with no clue on where they might be.