It is not necessarily a bad thing when an employee explores the job market. Usually, employees who meet with other companies will either find a better opportunity allowing you to find someone more engaged or they will realize they have a good thing and recommit to your company.
Under an employment-at-will relationship, employees are free to search for other employment opportunities. How you react will probably depend on how valuable the employee is to you and your company.
Please note that if there is an employment contract in place, things must be handled differently on both sides.
The Workplace Advisors is BSCAI's endorsed HR partner, providing HR support, recruiting and employee engagement services. For a link to our menu of services for BSCAI members, go to www.TheWorkplaceAdvisors.com/BSCAI