• the helicopter should detect all missiles that are currently in the air(or at least the pilot should receive a warning if the helicopter doesn't have a radar[AH/MH-9 or any transport helicopter in RL])
• if they are ir guided you mostly don't know if it is targeting you or the helicopter near you(it is important to decide by the radar trajectory to calculate where it is going... although today's system can predict the trajectory of the missile, so i'm not sure about that :P )
• you don't detect the ir locking mechanism
• you CAN defend from ir locking mechanism by dumping flares as precaution(awesome!)
• you can detect LASER warning(so we need laser guided missiles - http://feedback.arma3.com/view.php?id=9973)
• to defend from laser guided missile you need to outmanoeuvre it
• for RADAR guided - detects both locking and missile - then it is needed to dispense chaffs(for ARMA's sake I wouldn't mind that flares and chaffs would be dispensed at the same time always)

It would be so much more interesting to fly as the pilot!