After meeting with the informant Mallard, go to the largest section of ruin just past the building that you met him in. There should be a guard patrolling it and a few boxes of ammunition lying around. Once at this ruin, turn left so that you are facing towards the road that connects to the bridge. There should be a small section of wall, on which a Sr-25 SD is located. Getting this weapon will allow you to kill the guards and the sniper in the watchtower easier, which should prevent the alarm from being raised.