This is in fact how it has worked at many employers where I've seen this policy. It's a feckless policy, as most companies lack the resources to effectively search even the small samplings that are done for window dressing. So the policy effectively only disarms the compliant, assuring that any evil-doer will have fewer capable responders to deal with. The policy fails at both making the collective environment safer, as well as infringing on individual security. Only governemnt installations can afford the necessary real technology and manpower to effectively carry out these policies.
If I was an employer, I wouldn't ban my employees from keeping guns in their cars. It's a dumb policy, but employers are free to enact dumb policies.