Speak to Mikhail in the Wetlands.
A contact of mine retired in the Wetlands at the harbor city of Menethil. He should be willing to help on your word alone if you tell him you're working for me. His name is Mikhail. He mentioned becoming a bartender before leaving Stormwind a few years ago.
Speak to him, find this Slim, and find out if there's anything else to know about this kidnapped diplomat. We need to find out where they've taken him, or at least who he is. The docket doesn't mention anything about their plans after he's captured.