The risk of loss is yours if properly given notice. Do you know that it was thrown away or possibly the cleaning company personnel helped themselves? Perhaps best to contact them and inquire what happened to your property....making waves with your employer over something they did not assume any responsibility would be a mistake, especially if you were warned.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained. Please click "helpful" or "best answer" if my answer added any value or add a "comment" if you have more info for me to help you get a better answer.
Yes any item left in the sink will and can be thrown away. You were on notice. And, the policy may not be the employer's. That policy may come from the building landlord and/or cleaning management.
From what you stated it is very clear that you were on notice as to what would happen to anything that you left in the sink. Guess what? That is what happened.
No information provided in response to these questions can be relied upon in any way without further personally consulting with Attorney Kerrigan and Attorney Kerrigan consulting personally with you regarding your specific legal situation.