If your devices are approved for personal use by employees, they must log out of iCloud using their Apple ID on the device before returning the device. This will remove the activation lock created via the Find My iPhone (or Find My iPad) feature (see this link for more information Link):
To do this, employees follow the steps described at this link and select Sign Out in the Apple ID section of Settings:
The device must then be reset to factory settings in the settings using “General” > “Reset” > “Erase all content & settings” and is then ready for return.
If it is not possible for the employee to remove the activation lock and reset it to factory settings directly via the device, but the device is still in the device management of your MDM, you may also be able to do so as part of a remote wipe Remove activation lock – this only applies to supervised devices (for more information on support, please follow this link):
In this case, please consult the documentation for your MDM solution for the specific steps. Please note that devices with activation lock switched on cannot be used after return and may be subject to a damage fee.