Employment Law Questions? Ask an Employment Lawyer.
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Any property that is kept on the employer's property may be checked by the employer, which includes your tools. However, he cannot break the tool box, for that he would be liable for the damage and he can also be sued for any losses of tools you incur. While he is allowed to inspect the tool box, he cannot just randomly and continually break into it. If he will not stop, then you can sue him for the damages to your tools in small claims court.
Putting you down in front of customers is workplace bullying and not being civil, but under the law, I am afraid that is no legal case allowed for that. You have a case for him damaging your tool box and for any missing tools.