Bicycle detail

I rode my bike to my office at 71 Stevenson Street. This was the first time I had taken my bike to the office and I thought I would be allowed to bring it inside, but when I arrived the security guard explained to me that property management prohibited bikes inside the building. He instead offered to let me borrow a lock if I signed for it inside. I left the bike a few feet from me leaned up against a window while signing for the lock to borrow, and in the few seconds of it being unattended a man ran up, jumped on the bike and rode away. I chased him for about two blocks, but was unable to catch him (although I did get close and threw the bike lock at him - missed by a hair). Really big bummer as this was the first bike I've ever bought and I was just getting into cycling.