Bug:
The alarm indicating an enemy anti-air has locked onto the player's aircraft continues endlessly, should the player fly out of the threat's line of sight before it is disabled or disengages on its own.
It will continue to sound even after landing (e.g: for rearming) and stops only after returning to the area where the threat is located and either destroying it, or causing it to somehow lose focus without going out of range again.
Setup player as pilot of a jet such as the F/A-181 Black Wasp II
Fly it into an area where enemy anti-air (Tigris, ManPads, etc) are present
Continue until an enemy unit locks on to you, triggering the lock on alarm
Accelerate away with lock-on "bee-boop" tone sounding, use countermeasures to dodge any missiles fired by enemy
Speed your way out of the enemy line of sight while it still has a lock on you.
Observe if the alarm stops playing or not (sometimes it does, other times, it doesn't)
Also happens with helicopters, anything capable of detecting AA threats locking on and sounding out the alarm for it.
This has been observed in more than one MP Coop server, in various versions of "Invade&Annex" and "Domination" - It is thus reasonable to exclude mission-specific conditions as a probable cause.