If you change a unit from captive to no longer captive (for example, to "reveal" an undercover unit), enemy units that already have prior knowledge of the unit as a captive will only partially engage, making it very easy for the undercover unit. This seems to be a bug.
However other enemy unit the unit later encounters will engage freely with full force.
Lou Montana
Talkingtoaj's note does not apply to Arma 3 anymore (earlier than 1.76)
A captive unit will immediately be shot (after a normal "human" reaction time from the AI)
DreadedEntity
In A3 2.08.149102, entering an empty vehicle with the same side as the player as defined in configFile >> "CfgVehicles" >> _vehicleClassname >> "side" will briefly switch the side of the vehicle to the side of the player soldier (specifically, while the get in animation is playing), even if captive. Once exiting the vehicle, it will retain that side until the player enters a new vehicle or, seemingly arbitrarily, moves more than 100 meters away from the vehicle player distance _vehicle > 100. If the player gets within 100 meters of the vehicle without entering a new vehicle, it will again switch to the players side. This is probably a bug, in my opinion