When exactly does the data gets wiped out once bootloader is unlocked?
This may vary with the device, but on Nexus devices, as soon as you accept the bootloader unlock on your device after issusing fastboot oem unlock, the device is wiped so you would not be able to see the data in TWRP. This is evident because after issuing the fastboot oem unlock command to a Nexus device and confirming the operation on the device, the following text is printed to the terminal: Unlocking bootloader... erasing userdata... erasing userdata done erasing cache... erasing cache done Unlocking bootloader done!