That serial numbers are traceable to where and by who the device was bought? Nope, pretty sure everybody knows that. What this agent probably didn't know was that laptops aren't bikes and the serial number is available in more places. In OSX you can see it by simply going to the Apple menu > About this Mac.

I don't know how old the agent's MacBook was, but since at least 2012 most components have their own individual serial numbers that are associated with the computer. So even if someone too clever by half starts swapping the logic board your Apple computer will call home and get flagged if the numbers don't match.

He was convicted of stealing confiscated property. Maybe this computer was confiscated as well, maybe he stole it, and maybe it already had its serial number filed off.

How is that hardcoded into the device? It's not just on the drive somewhere. Is it changeable? Not something I'd really thought about before but maybe there's some way of changing it.

The serial number is burned into the motherboard. At least in the case of Lenovo laptops I've dealt with, it seems to be possible to modify. When we send machines for repair, if they replace the motherboard it comes back with the same serial number we sent in. In some cases, we've been sent systems without serial numbers and they refuse to boot.

My 2010 MBP came without a serial number burned into it. It was causing trouble when I tried to use things like Facetime from the Mac because apparently the protocol depends on this serial number. I had to download a utility and burn it to CD, boot the CD and flash a serial number of my choice. I was tempted to go with 0000000001...

I've had that after a repair (2007 MBP logic board replacement). Facetime refused to work without a serial number.

Usually its on the logic board.

Usually it's quite literally burned into a ROM chip.