Retrieve Previous Computer Serial Number From Old Ext Hard Drive
The serial number is read directly from the machine's logic board, so it won't by default be located in any file on the computer.
Did you purchase this computer from an Apple Retail Store? They keep receipts going back many many years in their retail systems and will happily pull a receipt for you if you can prove identity.
Likewise, if you registered the system with Apple upon purchasing (from anywhere) and can call AppleCare, they should be able to provide you with the serial number for your police report. Best of luck!