Say you have a computer with the system drive encrypted by BitLocker and you're not using a PIN so the computer will boot up unattended. What happens if an attacker boots the system up into the Windows Preinstallation Environment? Will they have access to the encrypted drive?

Does it change if you have a TPM vs. using only a USB startup key?

What I'm trying to determine is whether the TPM / USB startup key is usable without booting from the original operating system. In other words, if you're using a USB startup key and the machine is rebooted normally then the data would still be protected unless an attacker was able to log in. But what if the hacker just boots the server into a Windows Preinstallation Environment with the USB startup key plugged in? Would they then have access to the data? Or would that require the recovery key?

Ideally the recovery key would be required when booted like this, but I haven't seen this documented anywhere.