I can murder a HECU marine right in front of his friend, and his friend will stand there without looking around at all (literally just stand there and look at a wall or something). This only seems to happen when I kill a HECU without him being able to make a sound (spot me or shoot, etc.)
Here is a demo showing what I mean. Every HECU in this area should have gone on alert as soon as I murdered the first guy. Instead, they all stand there waiting to be headshotted