He might have bribed someone; some agent, etc. And got the no. Plates.
I would suggest to visit the excise department and take up this issue with them. May be they could guide as to what actually happened?
Secondly at the same time, you can request to block transfer of vehicle in anyone's name. And also lodge complain with the relevant authorities for confiscation of your ride. May be in this way, the current person driving your ride will rush to you for the resolution of the issue.
I hope it helps. Thank you.