A Trezor box is protected by the seals, and it is also virtually impossible to open the device without breaking it (the case is sealed using ultrasound), so you would see right away if someone had tampered with it.
All Trezor devices are distributed without a firmware software, so you need to install it for your first use. If a firmware has already been installed, then that means someone used the device before you.
The bootloader verifies the firmware signature. The device only runs if the firmware is correctly signed by SatoshiLabs.
Our Bootloader software is write protected, as the JTAG is disabled. The MCU is safeguarded by the Memory Protection Unit, which ensures that you're using the valid firmware version signed by us. If not, then a warning message appears on the Trezor display.
Because the recovery seed is generated during Trezor setup, there is no default seed; you will generate the seed for yourself.
If you encounter any Trezor device with preinstalled firmware, but it was purchased as a new device, please let us know.