In security design, a tamper-evident hologram or holographic seal makes an unauthorized access to the protected object easily detectible.
If the device's USB-C port is covered with a Trezor hologram and the device does not show any warnings at boot, it can be safely assumed that nobody modified the device during the transport.
Apart from the physical tamper-evident hologram, the Model T also uses software safeguards against tampering. The device firmware and bootloader are signed by SatoshiLabs, and these signatures are checked whenever you start the device. The Trezor will warn you if the signatures are invalid.