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If you can prove what you did to try to locate the owner of the found property, then in court they would not be able to prove that you had any intent to permanently deprive the owner of the animal. This is what they need to prove you are guilty of theft, intent to permanently deprive and your proof of trying to locate the owner shows lack of such intent.
If the police do not have any of the evidence of your efforts to contact the owner, then this could be why they would charge you for theft of the animal.
If you prove the owner/police knew that you went through all of this to locate the owner to return the dog and they fabricated the charges against you, then you cannot sue for defamation, but you can sue for malicious prosecution but you have to win your criminal case first before you can sue for the malicious prosecution.
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