When you first start, go on the other side of the flowers, but make sure you still see him. When you get to the last flowerpot, WAIT until he has turned the corner and is slightly past you. Then follow him on the sidewalk until the next corner. Next cross the street so that he can't see you, because he looks to his left. After he does that, cross the street again and follow behind him. Now you are in East Village. Stay along the upper side of the road until you get past Katie's store. Then cross the street. When you get to a tree in front of you, go straight up. Don't go to close to Thief V. Then when he goes on a tree-bridge, wait until he's past it and then go. Do the same for the next few. Next you'll come to a cliff-looking thing. Stay there! At this point, Thief V turns all the way around. So you have to hide. But don't wait too long, or else he could get away. Now you're close to his hideout. Don't get too close to him, and then, you're there!

Also, changing your clothes and hairstyle won't help. Because he wouldn't run specifically away from you. Even if a normal townsperson saw him, he'd run.

If you need more help search "MySims Agents DS Walkthrough Part 6". This is the part where you follow Thief V so that video might help.