Apple CEO Tim Cook displays his personal Apple Watch to customers at an Apple Store on April 10, 2015 in Palo Alto, California.

Tim Cook has been spotted at the campus test-driving a device that tracks blood sugar, which was connected to his Apple Watch.

A source said that Cook was wearing a prototype glucose-tracker on the Apple Watch, which points to future applications that would make the device a "must have" for millions of people with diabetes -- or at risk for the disease.

As CNBC reported last month, Apple has a team in Palo Alto working on the "holy grail" for diabetes: Non-invasive and continuous glucose monitoring. The current glucose trackers on the market rely on tiny sensors penetrating the skin. Sources said the company is already conducting feasibility trials in the Bay Area.

Tim Cook also talked about the device to a roomful of students in February at the University of Glasgow, where he received an honorary degree. He didn't say if it was a medical device from a company like or , or an Apple prototype.

"I've been wearing a continuous glucose monitor for a few weeks," he said. "I just took it off before coming on this trip."