Unfortunately, it's often internal theft by people who understand how theaters work/are set up/not heavily guarded.
As mentioned in the article, the audience and access to backstage are not too far apart. The theatre has riser-seating 3/4 around the stage and there's a weird little alley that runs nearly underneath two sets of the audience risers, leading to backstage. There is also a flight of stairs that leads to the booth and admin offices.
Who ever the crook is, I suspect he had some knowledge of the layout of the theatre and where the dressing rooms were. Whether he worked in the industry, or just knew someone who did is anyone's guess right now, but that's what I'm guessing.