If the engine is hot, the fan will continue to run to bring the engine back down into normal operating temperatures even with the engine off. This prevents the coolant from boiling over on hot days and helps cool off the A/C condensor proir to the next drive.

could be a faulty sensor in the cylinder head that activates the fan ,if this is a 99 their could eb a switch in the radiator but later cars use a sensor in the head with a relay