One of your best employees is leaving. You suspect he's leaving for a job with a competitor. How do you act?

Offer him additional benefits, salary or allowances that may take him/her back, but in the end we always know that employee have the rights to choose what would be the best for him/her.

If the employee is willing for a frank discussion, have it with involvement flexibility and leniency .  In the mean time review the situations that might have lead him to take such a decision.   If there is a positive sign from him to continue happily accept it, otherwise it  would be better to allow him to relieve with all his terminal benefits promptly paid.

Hire a detective or spy on him ! There are alot of reasons to leave a job for example the school of your kids or the new job of your wife in a different city or whatever reason we would have and it's non of anyone business to questioned us so Leave him/her alone or offer him/her a better salary.