We need to locate someone who worked at an apartment building office a year ago. Can any company's employee records be easily accessed for person-location purposes by the public or private investigators?

No business records are not public record and even a private investigator will not be able to access records of a private company.

Now you can go to them and ask them if they have a forwarding address. They may give it to you, or may not.

If you have their social security number, they can normally be traced though credit reports

If the person held any certification or licensing (like maybe a Real Estate agent's license), that's public; you can go through the agency that licensed them.

Thanks for the help.