According to Washington news station KIRO 7, charges were filed last month against a man who allegedly broke into Valve's office multiple times in June 2018 and stole "$40,000 worth of equipment and games."
The suspect, Shawn Shaputis, was previously arrested on burglary charges that same month, and arrested again after leading police on a chase in a FedEx truck. He is not currently being held by police, and there are multiple arrest warrants with his name on them. Another will be issued if he fails to show up in court this Wednesday.
The break-ins occurred at Lincoln Square South, which also houses offices for Unity, Epic Games, The Pokémon Company, and Samsung, according to the Bellevue Downtown Association.
A fact sheet from the Commercial Brokers Association states that Lincoln Square South includes a "24-hour security force with closed circuit TV surveillance." So how do you steal $40K worth of stuff in a building like that? According to police, the suspect wheeled a recycling bin into the office through a stairwell, filled it with computers and games, and then wheeled it to his parked car across the street, next to a Chipotle.
Police say they found unsold stolen items in the car, which is partly how they connected the burglaries to Shaputis. Police also claim he admitted to entering the building, and shared a GameStop security tape with KIRO 7 which they say shows Shaputis selling stolen games.
Whether any company secrets were stolen is unknown, but presumably Valve has devices containing sensitive material well-secured, so that physical access would not be enough to unlock their secrets.
I've contacted Valve to ask about the nature of what was stolen and what has been recovered, though it's unlikely the company will comment on the case.