I'm finding the Sentry and Guard commands not very useful. Most of the time a ship will only stay in either state for one turn. Next turn I'm right back to telling my radar picket scout to go into sentry. Looking in the manual.
• Sentry: Orders a ship to remain idle until another ship, friendly or enemy, comes near.
• Guard: Commands the ship to sit idle until an enemy ship comes near.
My suggestion is to have only an enemy ship cancel sentry. Guard mode seems to work better but it looks like unarmed ships will cause it to cancel. Only armed enemy ships within one or two movement turns should cancel guard mode.