Posing as a security guard or an IT person, he also breezed through Bay Area companies late at night looking for laptops and other computer equipment to steal, Field said. "He had a good eye for what was valuable," he said.

He sometimes gained access to different parts of the building by picking up security badges he found lying in unoccupied cubes, Field said.

If stopped by company employees, "he would talk to them as if he was completely justified in being there," Field said. "Like he was an IT person doing some work or a security guard making sure the place was secure."

"Being a former network administrator, he could talk the talk as an IT guy," he added.

Madrid even wore clothes that resembled a security guard's uniform, Field said.