It is likely that you are not being taken down by the Infantry Fighting Vehicle which can only shoot aircraft when they are painted.
If you receive no "locked on" warning and no "incoming" warning as well, then there is a good chance that you are being shot down by handheld Engineer's RPG rockets. When helicopters go below the level that they can be locked onto by the anti-aircraft tank or anti-aircraft missiles then it is fairly easy to shoot down a low altitude helicopter with a standard rocket propelled grenade.
The anti-aircraft tank has radar so that it can track and lock on to you but you will always receive a warning, especially when there's a missile inbound to you. The Infantry fighting vehicle with missiles can target you only when you have been painted, but again you will receive a warning. The engineers anti-aircraft shoulder held missile also will lock on to you and you will receive a warning for that.
I strongly believe you are being taken out by an engineer's RPG. They are long range and they are deadly, and you will receive no warning when you are hit by an RPG.
I get a warning when i am getting painted no warning when the IFV missile is incoming and no i am not getting killed by handheld launchers
