As others have said, if you have to leave a bike, then the best locks you can use.
We've had security issues with out new bike 'cage' at work. From locks that stop working, to folk leaving the door open to the magnetic lock not being fitted properly.
We have also has theives arrive tooled up with a portable angle grinder. They went for a new bike that had only been locked with a chain - they went through the padlock in seconds. The shelter is on CCTV, but they wore hooddies and were gone in seconds, way before security could get over.
I'm using a New York Fagh - yes its heavy, but about the best you can get. I'm also using another d-lock and two Abus Bordo's to secure the front. Fortunately, the thieves targeted the easy bikes to take. The down side is they know what other bikes are left in their all day.