No Country for Old Men

Terrell County, Texas in June 1980 is desolate, expansive country, and Ed Tom Bell (Tommy Lee Jones) laments the increasing violence in a region where he, like his father and grandfather before him, has risen to the office of sheriff. In a neighboring county, Anton Chigurh (Javier Bardem), a hitman, strangles a sheriff's deputy to escape custody and steals a car by using a captive bolt pistol to kill the driver.
While hunting in the desert, Llewelyn Moss (Josh Brolin) comes across the aftermath of a drug deal gone awry. He finds several dead men and dogs, a wounded Mexican begging for water, and two million dollars in a briefcase. He takes the money and returns home. That night, Moss returns to the scene with water. He is pursued by two men in a truck and escapes. At home, he sends his wife, Carla Jean (Kelly Macdonald), to stay with her mother, then drives to a motel in Del Rio, where he hides the case in the air vent of his room.
Chigurh, hired to recover the money, abruptly kills his employers after obtaining a clue to Moss's identity. Arriving to search Moss's home, he uses his bolt pistol to blow the lock out of the door. Investigating the break in, Sheriff Bell notices the blown-out lock. Using a tracking device hidden with the money, Chigurh goes to Moss's motel and kills a group of Mexicans who are preparing to ambush Moss in his room. Moss has rented a second room adjacent to the Mexicans' room with access to the same vent in which the money is hidden. He retrieves the briefcase just before Chigurh opens the vent and finds it empty.
While staying at a hotel in Eagle Pass, Moss discovers the tracking device, but Chigurh has already found him. Their firefight spills onto the streets and both are wounded. Moss flees to Mexico, stashing the case of money in weeds along the Rio Grande. His injuries are severe. He asks for help from some musicians, who take him to a hospital. Carson Wells (Woody Harrelson), another hired operative, fails to persuade him to accept protection in return for the money. Chigurh cleans and stitches his own wounds with stolen supplies and kills Wells at his hotel. Moss telephones the room and Chigurh answers. Lifting his boots to avoid Wells's blood, Chigurh tells Moss that he will kill Carla Jean unless Moss gives up the money; he remarks that he will kill Moss regardless of whether he receives the money.
Moss retrieves the case of money and arranges to meet Carla Jean at a motel in El Paso, where he plans to give her the money and hide her from danger. Instead, she reluctantly accepts protection for her husband from Sheriff Bell. Carla Jean's mother unwittingly reveals Moss's location to a group of Mexicans who had been tailing them. Bell reaches the rendezvous in time to hear gunshots and see a pickup truck speeding from the motel where Moss lies dead.
That night, Bell returns to the crime scene and finds the lock blown out. Chigurh hides behind the door. Bell enters Moss's room and sees that the vent cover has been removed and the duct is empty. Later, Bell visits his uncle Ellis, an ex-lawman, and tells him he plans to retire because he feels "over-matched". Ellis points out that the region has always been violent.
Carla Jean returns from her mother's funeral to find Chigurh waiting in her bedroom. She refuses his offer of a coin toss for her life, stating that the choice is his own. Chigurh leaves the house and carefully checks the soles of his boots, presumably for her blood. As he drives through town, he is injured in a car accident and bribes two young witnesses for their silence as he flees the scene.
Now retired, Bell shares two dreams with his wife. In the first, he had lost some money his father had given him. In the other, he and his father were riding through a snowy mountain pass; his father had gone ahead to make a fire in the darkness and wait for Bell.

Question: What did Moss discover while staying at a hotel in Eagle Pass?
Answer: A tracking device.