Dan and Fly:
Yes, and no. One way in which an employer could discover firearms in vehicles is what actually happened at my former employer. The emloyer got information that some people were dealing/taking drugs on the property. They either hired a private company, or brought in law enforcement (honestly, I'm not sure which) with sniffer dogs. I believe that is their right (to search their own property). The dogs were capable of finding drugs and/or firearms (explosives), and they hit on a number of vehicles. The owners of the vehicle were brought out to open up their vehicles, and some (at least one) had firearms inside. That employee(s) was fired. I believe the search would still be warranted, even under this proposed bill, and therefore, an employer might still find out about a firearm in a vehicle and procede to fire the employee without actually saying it was because of the firearm.
Yes, if there were a large number of employees carrying in their vehicles, the company would be hard pressed to fire them all for two reasons: first, they'd lose a lot of their work force which they would need to replace; and second, it would be fairly obvious to a reasonable person (legal criteria) that they were fired for carrying. When you look at the figures, though, it is highly unlikely that any significant portion of any particular work force would be carrying, and therefore it would be a relatively simple thing for an employer to fire the one or two employees who did carry for "other reasons".
To some extent, this would be just a "feel good" measure without the teeth to truly prevent someone from being fired for carrying on their employer's property. On the other hand, I believe there are many employers who only create the "no-guns" policy to placate their anti-gun employees, insurance companies, or their perceived public image (not to mention the law, as in school property). If they were "relieved" of this ability to dictate a no-gun policy, many of them may actually be happy about it.