You can tell a fair amount about a person based on their workspace. Most office workers have desks, but for athletes this space is their locker. Just like us, many of them keep photos of loved ones (or media members if you're Kirk Cousins), supplies/equipment to help them do their job, and random momentos.
Other teammates like T.J. Watt have speculated that there's a fridge back there. The eerie space even spooked WR JuJu Smith-Schuster, who, until recently, didn't even know the secret spot existed. One day Roethlisberger popped up, seemingly out of thin air, leaving JuJu wondering, "Yo, where'd he come from?"
This tale is just more proof that just when we think star athletes are like us, they prove that they're on a whole other level. If we go and add some sort of secret compartment to our work space, and people find out about it, we're likely going to have a meeting with HR.
Wait, who am I kidding, I don't even have enough space to keep anything secret at my work station, much less room to build a James Bond-esque compartment.