John Doe's car was stolen, along with a copy of the key. The theif then sells the car and key to someone else. John Doe is walking down a sidewalk, and sees his car in someone's driveway. He checks the VIN, and it is indeed the same car. He was carrying another copy of the key (only one of his two copies was stolen, and he never took this copy off his keychain), unlocks the car, takes the new "owner's" belongings out and sets them on the ground, and drives home in his car. He had never reported the car as stolen. Is this legal?
Assuming the car was really stolen, not "stolen" in the sense my friend borrowed it and never gave it back, you could absolutely recover the car. The reason you could, is because you can't steal your own property. Theft is very specifically the taking of ANOTHER's property. Here the property and the title to it has remained with you, because it was stolen. A thief never gets title in stolen property--it's called a void title. A void title is no rights at all as compared to a voidable title which has no rights against the true owner, unless you are subsequent transferee for value without notice (Bona Fide Purchaser).
Now we can even take this slightly further. Imagine our thief stole your car and now sold it to Hapless John and all the remaining facts are the same, can you still take the car? Yes, because title in property tracks from the seller of the property. You can only over transfer as good a title as you have in the property. So thief has a void title. When he sold Hapless John the car he transferred a void title, which is no interest at all. So when you see the car and take it back, Hapless John calls the police and reports it stolen too. As long as you could demonstrate it was stolen first, that's the end of the issue. Hapless John has to go find the thief and get his money from thief.
Now obviously this all assumes you see the car on say a street or whatever. If you saw it saw it in a driveway, you could end up being charged with trespass unless your state privileges the recovery of stolen property by peaceable means. You still wouldn't be liable for theft, because of the void title, but you can get the other charges.
All that said, you are better off having the police come and assist you. If you are even slightly wrong you get hit with the full force of the consequences. It's better just to have a police officer come and assist from a criminal liability stand point.