Possibility of flashing a new ROM to a stolen Nexus
It is possible to use the phone without flashing a new rom, or a bootloader unlock.

The thief can reset your phone using the recovery mode


Recovery mode can be entered by powering down the device and then pressing the volume down and power button simultaneously.
then he will have an option to wipe data/factory reset the phone.
Upon reboot he will be greeted with Factory Reset Protection.
The FRP can be bypassed on 8.0 oreo using the method here