I just won a battle before taking the first screen snapshot below.  There are no enemy ships anywhere near my fleet, but the Sentry option is disabled and the popup says "There is already an enemy ship within sensor range".  If I just move my fleet one cell, the Sentry option becomes enabled as shown in the second screen snapshot below.

I believe the issue is that the Sentry capability for the fleet that wins a battle is not evaluated immediately after the battle.  Moving the fleet one cell triggers the Sentry capability reevaluation.