When you bought a car, the procedures were complete, but when the driver bought your car, did he even steal your procedures? Do you need to go through the corresponding procedures (such as car purchase invoices, etc.) when buying, selling and transferring vehicles? )? Of course, he can sell it on the black market without any formalities, so he is a theft suspect. You don't need the owner's signature to sell cars on the black market.
I suggest you report the case immediately and ask the driver to return the car and compensate for the economic losses. The law will deal with it according to the driver's later performance.