That's a fairly trivial check to make. I would be fairly shocked to find out that people have discovered multiple sidechannel attacks against Bitlocker to leak

That's a fairly trivial check to make. I would be fairly shocked to find out that people have discovered multiple sidechannel attacks against Bitlocker to leak keys out (if you have physical access to the machine while the disk is decrypted), and didn't manage to catch the fact that the key was getting sent over the network. The more plausible potential for Bitlocker being broken is that there is some subtle flaw in their crypto implementation.