On Thursday morning, a top executive at self-driving truck developer PlusAI received an unexpected congratulatory text message from an acquaintance. “I have been proud of you for your adventure and success at Plus. Just saw two of your trucks near my work place! :)” the message said.

But PlusAI’s trucks weren’t supposed to be near this person’s office, in Newark, California. As of the last the company knew, the trailers were docked outside its warehouse, half a mile away. Soon, according to a police statement, cops were on the scene to help recover the two trailers, emblazoned with logos for PlusAI and Nvidia, the most valuable company in the world. The two companies are working together on AI software and autonomous truck technology.

That’s when PlusAI employees gave police a hint as to why whoever hooked their own cabs up to the trailers and drove away might have abandoned the whole thing after breaking them open: Each trailer was full of about 20,000 pounds of sand.

“PlusAI uses simulated loads in its trailers to assist with research and development testing,” says Lauren Kwan, a company spokesperson. The trailers were recovered with “40,000 pounds of sand intact.”

PlusAI’s tech-enabled truck cabs—where the driver typically sits and controls the vehicle—were parked inside its warehouse and weren’t taken, Kwan says. The trailers were secured with hand locks, which were broken.

The case is under investigation, and no arrests have been made, says Amy Gee, a spokesperson for the Fremont Police Department.

Nvidia didn’t immediately respond to a request for comment.

The incident comes at a time of rising theft targeting high-value computer equipment, especially chips and other electronics meant for use in data centers. In August, WIRED revealed two California incidents in which thieves deliberately struck the vehicles of security escorts that follow high-value tech loads to ensure they reach their destinations. The trucks involved were then driven to another destination. As of August, millions of dollars of data center equipment onboard had not been recovered.

This year, thieves have also targeted Tesla batteries and various bitcoin mining machines. Authorities have arrested suspects in some cases, but many crimes remain unsolved.

Some of the most common targets of cargo theft in recent months across the US include metals and enterprise-grade computer and networking equipment, according to Verisk CargoNet, an analytics and risk assessment firm.

This story has a happier ending. By Thursday afternoon, the sand trailers were back with PlusAI, the Fremont Police Department said.