When you reach the “!” inside the tunnel you run into Osman - he was the one of Rais' men that we saw getting his hand chopped off a long time ago when we first met Rais and visited his HQ. Osman is now hiding out in the Tunnel - his brother who was also one of Rais' men had helped him escape but has now gone missing. Osman wants our help in finding his brother Nazim.
Head for the market that is marked on the map and use the door to zone into its Instance, where you will find a set of keys in the bathroom. In the office whose door you have to pick there is a duct you can enter - be careful of the Bomber below the ducts as it WILL blow if it detects you.
If it blows and sets off the fire sprinkler there is a control back in the hall you first entered the market in - just head back there and reset it to silence the alarm.
Drop down and clear the infected and then head into the small room in the corner to talk to Nazim - he gives you a message for his brother. You leave and head back to deliver the message.
When you emerge you are literally surrounded by an army of infected thanks to the fire alarm!
As you arrive you run into a patrol of Rais' men - you take them out and then talk to him to deliver his brothers message. You collect your reward - and complete the side-quest.