Nexus 6P bootloader e-fuse remembers if hardware was ever unlocked | Pocketnow
QFuse on N6P

When unlocking the bootloader for the first time, the phone reportedly marks an area of one-time programmable memory with a flag that indicates the hardware’s been unlocked. The presence of this “electronic fuse” is visible above in the Qfuse section of the Nexus 6P’s fastboot menu.
And much like a physical fuse, once that e-fuse is blown, it’s blown for good: no matter what you do, restoring factory images and re-locking the bootloader, the Nexus 6P’s going to remember that it was previously unlocked.
QFuse on N6P