How does Tessa know there is a traitor/spy from Cheliax in the Shackles? I don't recall seeing that in the AP. I list below what I came up with, but I'm happy to use what is in the AP or someone else's idea if it's better. I could see my players asking her how she knows.
Tessa has a few spies in place among the clerks of the Chelish navy. These clerks have seen plans for a planned combat expedition in various order ledgers and ledgers for outfitting ships. The senior brass in the navy know the target, but Tessa's doesn't have spies among the senior brass. Nevertheless, her clerks are smart (Tessa would only choose smart spies) and they love her.
Based upon the amount of food/water provisions being planned for the clerks can estimate the travel distance of the combat expedition. That immediately reduces the number of possible targets to a handful - and The Shackles is on this short-list. Second, the expedition will of course be well armed. However, when trade-offs are being made between seaworthiness/speed/maneuverability vs. more firepower for this flotilla, the former is prioritized. Tessa's spies surmise that is exactly the choices one would make if they needed to sale near a hurricane and then fight speedy, lightly armed pirates. So it isn't like Tessa's spies are 100% sure of what is going down, but they are making logical conclusions around the information they do have.
When Tessa got this information, she thought why now, as she knows the storm, and the pirate's ability to fade into the islands has previously held back Cheliax. Tessa has come to the conclusion that Cheliax must have a traitor among the pirates of the Shackles, and access to that traitor has sufficiently bolstered Cheliax's confidence that they would plan an attack. So similar to the clerks making a reasonable assumption about things, Tessa is doing the same. In both cases, the clerks and Tessa are right.