We have a small shared kitchen space in our office. Lately someone has been leaving dirty dishes in the sink. We have a sign posted saying that everyone should take care of their own dishes and not leave them in the sink overnight. Unfortunately someone has been ignoring that common courtesy and not cleaning up after themselves. Our policy (stated on the sign) is that the dishes will be thrown out if left in the sink overnight. Apparently that has been done several times lately and dirty dishes continue to show up. The manager of another department that also uses that sink came to me and said they believe that one of my direct reports is the person responsible and asked me to handle the situation. One of their direct reports who sits in an office with a view of the sink reported to the manager who they think is the responsibile person.
My question is: what is a good way to handle this situation given that I have not personally seen this person leaving dishes in the sink?