Microsoft have released a fix to stop the Surface 2 prompting for BitLocker key. I have only experienced this problem once on my Surface 2 and its annoying, so I imagine it happening every time you restart being very irritating, the BitLocker key is a 64 digits long and you have to get it from Microsoft supports page.
The fix stops the Surface 2 prompting for the key and is available via Windows Update. This update only affects the Surface 2 and not the Surface RT or Surface Pro/2.
Details from the Microsoft support page.
Surface 2 prompts you for the BitLocker recovery key when you restart the device
When you use a Surface 2 device, you are prompted to enter your BitLocker recovery key after you turn on or restart the device or you resume the device from the sleep state. These messages may be random or they may occur every time that you try to restore the device to operation.
This update is available from Windows Update
Note This issue does not affect the Surface Pro 2. This update applies only to the Surface RT 2.