According to this arcticle, the plane was sold, so it was no stealing. However it may or may not have had an active registration with the Lithuanian authorities, which may have necessiated the smuggling to Bulgaria, skipping all the transfer permits and red tape before getting a registration in Bulgaria.
What is really interesting is that even though the contract was in a foreing language, the seller and buyer name must be recognizable on it. It leaves a bit of doubt about the honesty of the Lithuanian owner, when he said he could not remember the name of the buyers. Perhaps they didn't sign an contract, just gave him lots of cash, and that was it!
Still leave the question, why were the buyers in such a hurry.
https://www.thedrive.com/the-war-zon...-crew-vanished