I don't know Macs, but dell tag numbers are burned into the BIOS. Possibly checking that instead of a sticker that could have a fake serial is a more reliable method of verifying the number. i.e. your less than technically competent thief might not know about this or how to change the BIOS one.
> Derek tells the woman the C-number is off. Not the serial number—they were looking for some other number in the computer.