Pull the trigger before they're set and you run the risk of missing - and becoming a target yourself. There's a visceral thrill in getting through locked doors if you choose the testosterone-laden path. Blast your way in. You'll alert everyone in the area to your presence, but you won't have to slow down. Moving with stealth isn't always easy.
Pigeons are as skittish here as they are in the real world.
And when soldiers see a flurry of activity from birds that were fine a few minutes ago, they figure out where you're hiding. As for getting through locked doors, you'll have to pick locks, hack power modules and find other covert ways of breaking and entering if you want to stay under the enemy radar. Needless to say, neither of these styles of play is rigid. You can easily shift from one to the other, stunning people when they're alone and shooting when there's a crowd. And your hacking and lock-picking agility is something you determine as you spend upgrade credits.