Smugglers' Tunnels

A network of tunnels below the streets of Waterdeep, connecting the basement of The Yawning Portal to several other locations. They're in active use by smugglers.

(Unofficial name.)

One entrance is in the basement of The Yawning Portal, with a secret door concealed to look like an ordinary cask of ale.

There's no way Durnan doesn't know about these. It seems likely he's actively involved in the smuggling. If so, the fact that the City Watch are reluctant to hang around the Portal suggests that Durnan has a standing arrangement with them - which in turn suggests that he has significant leverage over them. "Friends in high places," as it were.