No, employers do not need to provide security for personal belongings; you bring your belongings to work at your risk.However, if they knew of thefts and didn't disclose them, that failure to disclose could potentially make them liable, if you sued them, under the theory that they were negligent (careless) in not warning, since their failure to do so prevented you from taking action to protect yourself, such as by not bringing valuable to work. You would have to sue, however, to try to recover this, if they don't voluntarily compensate you. Also, if they know who did the first theft(s) but kept that person on, in a position to steal more, that could make them liable under a theory of negligent supervision.