As soon as you move the TRMB or as soon as you get involved in any fighting with the imperial units the alarm will sound. The alarm will NOT sound when you capture the turrets. To be honest, though, once you've got a couple of turrets and a few of the locals with are you (a couple have some captured AT-ST's)your force is strong enough without having to worry about the alarm anyway.
I think that this is right, but don't talk my word as gospel - it's been a while since I did this level!
"Life. Don't talk to me about life." (Marvin the paranoid android).