HOW TO START THE SIDE MISSION
You have to make contact with the glitch. You do this by finding one of the glitched ad panels. This can be one on your first or second visit to prague. Look for these panels near metro stations.
MEET HELLE
Travel south and go to the electronics store new Koller's place. Go into the basement, message with Helle and agree to the mission.
TOURISM CENTER
Go to the tourism center and go in through a vent at the side building or from the upper window. There are three enemies inside. take them out and grab the pocket secretary from one of them. This will be important in the next mission step.
Enter the code from Helle into the panel for the locked door and pick up the disk in the room.
FUTURE-PAST ANTIKY
Travel north and go to the Future-Past Antiky shop. Talk to the clerk and press him about the store until he pulls a gun on you. Then bring up the pocket secretary you found earlier and he will give you the data reader.
ADAM'S APARTMENT
Go back south once again and go to your apartment. Use the data reader on the TV table and talk to Helle.
At this point you have finished the side mission, but you get attacked by a few enemies after the conversation. Take them out and you're good to go.