Hey @PR_60168 thanks for contacting us here, even if the original owner of the device isn't a Three Customer currently they would still be able to request the unlocking code by filling in the Unlock My Phone Form Here.
We don't support third parties unlocking devices however the original owner would be able to request the unlocking of the broadband device at the form linked above.
Thank you for replying, this is unfortunate. The account owner is no longer contactable. so another fully functioning device has to be thrown into waste.