Follow him and use the blowtorch.
When you are in the vents there will be red bars between you and the thief. Use the blowtorch to cut through them and climb up the ladder. Then go to your right, and do the same thing. Once you end up near the machine where the elevator and escalators work, you will see him again. Once again you have to use the blowtorch on the bars. When you catch him in the air vent chamber, he knocks you away.
Go back right to the vertical shaft, burn through the brown bars, and destroy the A/C control so that he has to move. Go back into the air chamber, across to the spinning fan on the left side, and burn through to Vent B. When he blocks your path, go left onto the dark maintenance ladder. When you catch him, you both fall out into the atrium fountain.
Eddie the snake saves the day, and the cops are waiting for the thief.
a baseball card
When you have to break into the empty store, Gus tells you there is a blowtorch in the manager's office (behind the plaque). Use it on the padlock, then follow the thief when he goes into the ceiling. You can use the blowtorch as you pursue him through the vents.
He is out to steal a rare baseball card from Slugger's Keepsakes.
The rare Honus Wagner baseball card from the safe at Slugger's Keepsakes.
There are no coins used on Night watch island.
When he is in the security cart, Gus will tell you how to get the other scooter (Segway) from the manager's office. Chase him up the left escalator in the atrium and he will crash. When he steals the card and flees through the air vents, follow him by using the blowtorch to burn through the grates (red and brown). You will also have to burn up the A/C control for at least one of the two vents.
night watch island
you use it to open the new store to catch the thief on night watch island the security guard tells you it's hidden behind the employee of the month portrait
You have three close calls: once in the Costume store, once in MacGuffin's, and finally in the empty store. But you do catch him in the vents after using the blowtorch to cut your way through the metal bars. You can't stop him in the large vent chamber (he tosses you away), but if you cut off the A/C controls, you can trap him. (see the related question)
Back Lot Island (April 2013)
Not the back room of MacGuffin's, but the Employees Only room at Good Deals next door. Use the toy robot at the camera to keep the lights on.
The thief drops the key in MacGuffin's but it doesn't fit the back door there. Go next door to Good Deals and try that door. You will need to use the toy robot to keep the lights on.