Well nothing is 100% secure.  Now that we have that out of the way is your data accessible, unlikely.  Without a dump of the memory from which the key can be extracted it would likely take someone years to try and brute force the encryption on the drive and the same could be said about using the computer for banking and other authenticated sites.  If the passwords are saved on disk there are tools in some cases that could be used to recover those, but they would have to get through bit locker first not to mention the fact that you changed your passwords.
It's more likely that if someone decided to keep the laptop that they would just wipe it out and install another operating system rendering your data irretrievable.