On a mac (osx/macOS), the serial number is usually not stored on the disk, it is stored in the firmware and available either printed on the backside/underside of your mac/macbook computer or accessible via software on a booted system using 'About My Mac' or System Profiler.
On recent versions of OSX, there are however a few system databases that store this information and make it available for forensic investigators to use (or for verification). These are:
All the above files are sqlite databases located in the 'root' user's Darwin user cache folder located under /private/var/folders/zz/zyxvpxvq6csfxvn_n00000sm00006d/C/. This location should be the same for all OSX/macOS installations (10.9 & above) because UID and UUID of root is same on all systems and does not change.
For more information on Darwin folders, see this blog post.
|Screenshot 1 - Table 'TableInfo' inside consolidated.db showing Serial Number
In the above screenshot, the serial number is seen starting with 'VM'. It starts with VM since this was a virtual machine; for real machines, you will see the actual hardware serial number here. I was able to verify this on several macs running osx 10.9 to 10.12.
In addition, other software might retrieve and store this information too. One such software, is KeyAccess, installed by Sassafras asset management system. KeyAccess leaves behind a binary file /Library/Preferences/KeyAccess/KeyAccess Prefs which also contains the serial number.
Another place where you might find the serial is sysinfo.cache. This is created by Apple Remote Desktop and is found at /var/db/RemoteManagement/caches/sysinfo.cache.