[X] Activate the Guard Reserves. Second Prince will see Xue Chi's action as overreaching, and you don't know how many reserves are actually still available, but if you can sell the story of your fears to Xue Son, you will have more protection.
[X] Investigate the redeployed guards, and discern how many reservists are available to be called up. Second Prince will remain in the dark about your suspicions, and you can decide to call up the reserves after the investigation.