Yubikey 2.2 or later has a non-alterable, factory programmed unique serial number which is different than the public identities configured.
Some YubiKeys do not come with their serial number enabled and readable bu USB. In these cases your key will show up in the list of USB devices, but will now show (More secure: has serial number) next to it. If this is the case, you must enable the YubiKey serial number:
Open the YubiKey personalization tool.
Select Settings
In the Serial Number Visibility section, enable USB descriptor
Please note that the YubiKey must be removed and reinserted after this option is enabled in order for the operating system to recognize the updated device descriptor.
For additional information, please refer to YubiKey Personalization Tool user guide