Home Alone 3

Peter Beaupre, Alice Ribbons, Burton Jernigan, and Earl Unger, four internationally wanted hitmen working for a North Korean terrorist group, have stolen a US$10 million missile cloaking computer chip. The thieves put it inside a remote control car to sneak it past security at San Francisco International Airport. However, a luggage mix-up occurs, causing a woman named Mrs. Hess to inadvertently take the thieves' bag containing the remote control car while returning home to Chicago. The four thieves arrive in Chicago and systematically search every house in Hess's suburban neighborhood to find the chip.
Meanwhile, Alex Pruitt is given the remote control car by Hess for shoveling snow, but she lectures him for scratching numerous itches. He returns home and removes his shirt to discover that he has chickenpox, and therefore must stay out of school. While recovering at home, Alex uses his telescope and discovers the thieves on look out for the chip. Wondering what the thieves want with a remote control car, Alex opens it and discovers the stolen chip. He informs the local Air Force Recruitment Center about the chip while asking if they can forward the information about the chip to someone.
The thieves conclude that Alex has been watching them and decide to pursue him. As a snowstorm hits Chicago, the thieves block off the road to the house, and Alice duct tapes Hess to a chair in her garage and leaves the door open. By this point, Alex has armed his house with booby traps and prepares to set them off with his pet rat, Doris, and the loud-mouthed pet parrot of his brother, Stan. After several break-in attempts, the thieves manage to make it into the house, despite falling into Alex's traps. They begin to search the house to find Alex. Alex runs to the attic and goes into the dumbwaiter down to the basement, and runs outside and calls to Alice, Jernigan and Unger. The thieves see Alex and notice a trampoline below them. Jernigan and Unger jump to pursue Alex, but the trampoline gives way and they fall into a frozen pool. Alice wriggles her way into the dumbwaiter chute, but falls down to the basement because Alex removed the bottom.
Alex rescues Hess and is cornered by Beaupre, but manages to scare him off with a fake gun. Meanwhile, the Federal Bureau of Investigation goes to Alex's siblings' school after being tipped off by the recruitment center. Alex's family brings the agents to their house, where the police arrive and arrest Alice, Jernigan, and Unger. However, Beaupre flees to the snow fort in the backyard. The parrot drives the remote control car into the snow fort and threatens to light fireworks, which are lined around the inside. Beaupre offers a cracker, but the parrot demands two. Since he only has one, the parrot then lights the fireworks, and flees. Beaupre is discovered and arrested.
In the epilogue, Alex and his family celebrate with his father returning home. Hess, who befriends Alex after he successfully rescues her, is there along with the FBI and the police while Alex's house is being repaired. While the thieves are having their mugshot photos taken, they seem to have caught Alex's chickenpox.

Question: Who do the thieves conclude has been watching them?
Answer: Alex