I saw tire tracks on the US side. What happens is probably that if anything, even a bottle cap flies from the Mexican side to the US side through the chainlink fence, the motion sensor picks it up and a vehicle is sent to investigate. 

You can reach a few centimeters to the Mexican side with your hands from the US side but I wouldn't be surprised if the patch of land adjacent to the fence were not constantly lit and under surveillance making it impossible to pick up anything from the Mexican side, put it into your pocket and walk away without being searched by border guards.

Thank you, and greetings to your family.