You will fall in a hole. You must solve two or three riddles to catch up to him. However instead of being behind him, you will be ahead of him.
Gametip
Choose "the bottom of the lake". Tabor and Katie will make you a diving suit for $2000. Go to Lea Lake. There will be several puzzles to solve to get you ahead of Thief V. Spade=50. Answer the riddle and move the statues accordingly into the four squares from left to right into the corner squares. Egg, chick, chicken.
How to follow Thief V without him seeing you
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.
Make your Sim sleepwalking
This is the coolest glitch ever! Just follow these steps:
You are going to need these in your house; a chair,table, and a lamp (anything that you can put onto the table).
Place the object and the chair somewhere in your house.
Make your sims sit on the chair.
Wait until they go to sleep.
While they are sleeping, go to the furniture icon and move the object on to the table.
Now get out of the furniture icon and you will see zzz's coming out of them.