There are various attacks against BitLocker and other forms of encryption. The most successful involve reading the computers RAM and extracting the key directly from memory. This usually relies on the computer being booted up or in sleep / hibernate mode when the attacker can get physical access to the machine. If the computer is fully shut down an off these attacks are much harder.

This video demonstrates the process

Even using a TPM module does not protect against this form of attack.

More information can be found on this link: