Yes, within a claim, theft is permissible, and chests are vulnerable to non-trusted players. Therefore, it is essential to take measures to safeguard your belongings. However, if any player resorts to dishonest methods or exploits to steal from your chest, please report their actions.
As a precaution, you may consider placing trapdoors around the chest you wish to protect. This will effectively restrict access and prevent unauthorized individuals from tampering with your belongings.