Enemy A.I. will not react to footsteps from behind while the player is in enemy disguise when there are their team members present.
When They See You In Disguise
When there are less than 4 enemy A.I. together, they will react to footsteps nearby and will be alerted.
Noticing A Missing Teammate
When enemies spotted you while not in disguise they will fire upon the player. Afterwards they will alert their own teammates by radio, walkie talkies, or an alarm.
Carrying The Wrong Equipment/Damaged Uniform
Enemies will not question your actions unless they notice the player not cooperating . They will either ask what you are doing or ask for your I.D. Disguise will work if the outfit covers the player from head to toe. If the head is shown then, the player must distance themselves from them before anyone gets a good look up close.
When they noticing an absent member, they will try to radio in. If no response, then enemy squad will be alerted.
If you are seen carrying something enemies don't have, they will question the player, where did you get that gun. Only solution is to use their equipment and wear the outfit in perfect condition and no visible wounds during stealth levels.