When customers show up at your business after you have closed, give them information in order for them to contact you. These customers may be off the street, truck drivers making a delivery, vendors making scheduled stops or anyone interested in your business and unaware of your posted hours. Post this information directly on the front of your business or front door. Enter in readable letters your name, address, telephone number, fax number and website address in order for those potential customers to call you when you are open. You may even want to add an email address for general inquiries. If you have an answering service, make sure they monitor incoming emails in the event they are urgent or require immediate attention.
This is simple, very cheap and the least you can do for your customers when running a business. Put this information on your front door (white letters on a dark shaded tinted glass, a neat clean sign next to the front door, etc.).