If the devices are not centrally managed by an MDM platform and you permit your staff to configure the device with their own (possibly private) account credentials to set-up and use devices, there is a very real risk that you’ll lose the ability to access or use the device if the employee forgets their Passcode or AppleID credentials.
Similarly, if the staff member were to leave your employment without properly unlocking the device, as a business you might totally lose access to (and use of) the unmanaged device. If you don’t have robust exit procedures, this is a real risk.
If the device is disabled or Activation Locked, you will require your original purchase documentation that proves ownership - usually the sales invoice. Documentation must clearly show the Serial/IMEI number of the device. With this documentation, Apple may at their absolute discretion assist with unlocking.
If you have the necessary documentation, Apple have introduced a portal though which you may be able to request release of an Activation Lock:
How to remove Activation Lock - Apple Support & start an Activation Lock support request.