A thief steals a car right before a natural disaster occurs, which would have certainly destroyed it, then later he returns it to the owner (or gets caught and the car is retrieved intact).
Would the fact that his actions saved the owner's property help the thief in his defense?
... however, in this case, it depends on which of your examples we use. Even though you use the word "thief", a person who intends to return the car to its owner is not one.
In england-and-wales, Section 1 of the Theft Act 1968 states (see my emphasis in bold):
(1) A person is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it
If the intention is to return the car to the owner, then this will not satisfy the need for an intention to permanently deprive. Therefore, they would incur no criminal liability for theft if, for example, they only took the car to protect it from the natural disaster.
However, you also mention the scenario where the thief is caught and the car is retrieved. If the requirements for theft are met, then the condition of the car is no defence to theft. In the absence of any actual defence, the perpetrator will be convicted accordingly.
The crime is the taking of the car - not any consequences that flow from that.
For a recent example, Sam Bankman-Fried illegally used investors funds and was sentenced to 25 years in prison and forfeiture of $11 billion. This is in spite of the fact that the illegal use paid off and investors will receive a return of $1.19 in the dollar.
That the purported victim benefited is not a defence in Canadian law.
For some offences, harm to a person is a required element. For those offences, if the required harm cannot be proved beyond a reasonable doubt, then the offence is not proved.
That is not a "defence" though — that is simply the prosecution failing to meet their burden.
It would not be a defense to the charge. But, it wouldn't be irrelevant either.
Prosecutors exercise discretion when deciding whether or not to press criminal charges, and in considering what kind of plea bargains to offer. A good outcome for the crime victim, or voluntary efforts of the perpetrator to remedy the harm done in the crime, can influence how a prosecutor exercises that discretion.
If the victim doesn't feel very aggrieved, the victim could also ask the prosecutor not to press charges, although the prosecutor wouldn't be required to honor that request.
Also, while guilt or innocence at trial is a binary decision that is usually made by the jury in the U.S. (although sometimes it is made by a judge in bench trial) according to inflexible jury instructions, there is a great deal of judicial discretion in making a sentencing decision after there is a conviction (if there is a conviction).
The amount of harm that the victim has suffered, and the attitude of the convicted defendant, are two important factors in how sentencing discretion is exercised.
In federal criminal cases, this sentencing discretion is codified in the United States Sentencing Guidelines which consider the dollar amount of the loss from the theft, and acceptance of responsibility, which together with the charge of conviction and the criminal record of the defendant, are used to generate a presumptive sentencing range, which the judge may then honor, or may deviate from for reasons described by the general portions of the federal criminal code.
In most state and local courts, the judge has much more sentencing discretion, with a range of options established by the offense of conviction and any aggravating factors (like prior criminal convictions) set forth by law.
If the harm suffered by the victim was modest, and especially, if the defendant tried to make things right after committing the crime in a way that causes the harm to the victim to be modest, this would often lead a judge to impose probation where a sentence of incarceration would be another sentencing option, and to impose smaller fines or short sentences of incarceration, if these are imposed.
While a scenario like the one in the question where an unexpected windfall event causes the harm caused by the crime to be much less than would ordinarily be the case, this also makes the crime less serious in many cases.
For example, the maximum statutorily allowed sentence for theft often depends upon the dollar value of the loss suffered (although more often, it depends upon the dollar value of the property stolen), and a lower value could make the difference between petty theft and grand theft, which would be the difference between a felony and a misdemeanor.
But even though the dollar value of the property stolen might be the statutory measure of the severity of a theft crime, if the vehicle was returned undamaged not long after the theft and the actual loss was only, say $300, rather than the $30,000 of loss that would have been experienced if the car was never recovered, the prosecutor might very well accept a plea bargain down to petty theft, especially if the vehicle was voluntarily returned undamaged before the defendant was caught.
Also, as another answer has noted, theft is taking property with an intent to permanently deprive the owner of it. If a vehicle were taken during a natural disaster with an intent to reach a point of safety from the disaster and then to promptly return the vehicle to the owner, that wouldn't be theft of a motor vehicle. It might be a lesser offense (like joy riding), or it might be no crime at all (if the offense could be justified as an action reasonably necessary to save one's own life or the lives of others).
A real life example of this is the case of Jabbar Gibson who stole a bus to help rescue dozens of people who were in grave risk of harm from Hurricane Katrina and drove them to safety. Rather than being prosecuted, he was hailed as a hero.
The action of stealing the car establishes the guilt without needing to investigate into events that occur afterwards.
The sentence will depend on the offender's actions (obviously, "returns the car one day later of their own volition" is a better look than "is caught by the police and the car is returned"), or economic/social/familial situation, but the victim impact is supposed to be irrelevant. ("Supposed to", because in practice, the criminal and civil trials are often one and the same, and judges are human too.)
It is hard to prove a negative but see for instance this section of the penal procedure code, which gives zero weight to victim impact in determining criminal sentences.
The victim of theft can sue for compensation (*). This requires them to prove (1) damage, (2) faulty conduct by the defendant, and (3) a causal link between the two. (The most famous article of the civil code is article 1240.)
A finding of guilt in a criminal trial satisfies prong (2). Prong (3) is easy to establish on the factual hypothetical offered here ("I did not have my car, because it was stolen from me, by you"). But prong (1) is greatly attenuated if the car was returned in short order, regardless of how this came to be. Liability is still nonzero because missing the car for a few days can have a detrimental impact on the plaintiff (maybe they had to pay for cab rides, for instance).
(*) In practice, they would often add themselves as party to the criminal trial (constitution de partie civile), rather than start their own action.
