A picture says a thousand words, so here we go.
Case A:
"Hicks, did you hear that?"
"Hear what, private?"
"There's someone moving in that treeline."
Corporal Hicks took aim at the treeline, but didn't see anything. The rest of the squad formed a perimeter and started scanning their immediate surroundings. Suddenly, a bullet whizzed through the air and hit private Hudson's vest, knocking him down.
"Where the fuck did that come from?" - yelled Hicks.
"I've no idea, we have to get the hell out of here!" - replied Vasquez.
"House, 12'oclock, move!"
The marines took cover, located the enemy after receiving more incomings and finally started fighting back.
Case B:
"Hicks, did you hear that?"
"Hear what, private?"
"There's someone moving in that treeline."
Corporal Hicks took aim at the treeline, but didn't see anything. The rest of the squad formed a perimeter and started scanning their immediate surroundings. Suddenly, a bullet whizzed through the air and hit private Hudson's vest, knocking him down.
"Target aquired!" - yelled Hudson immediately after getting on his feet and starting to put some rounds in the targets hiding behind the junk opposite the aforementioned treeline.
Some bullets did impact the ground around him, sonic cracks and everything and in a matter of seconds the entire squad was returning fire.
"Let's rock!" - Vasquez's war cry pierced the air as she cut the enemy down with her LSW.
"Yeah!" - replied Drake instantly blowing the head off the guy who just barely missed him a moment ago.
... and so the marines repelled yet another illegal alien incursion from across the border.
Now, which one do you preffer in FOnline? Auto-return fire/autosentry is a bad idea, as it removes any pressure on remaining aware of the situation and your surroundings, therefore reducing the surprise effect of any well executed move. On the other hand, autoattack with melee and unarmed is a good idea.